Why Is Gas 85 Octane in Colorado? The Rule and the Price

Regular gas can be 85 octane in Colorado and its mountain neighbors. The national model grade-name rule, NIST Handbook 130, sets the minimum antiknock index for “Regular” at 85 in the areas that the industry fuel specification, ASTM D4814, designates as altitude reduction areas, against 87 elsewhere. That is what the model rule says, and this page does not show that it is the rule Colorado applies. The handbook’s own adoption survey marks Colorado’s fuel regulation as in force but not based on the NCWM standard. Colorado’s statute, C.R.S. 8-20-204(1), makes the most current applicable ASTM specifications binding, subject since August 12, 2026 to exceptions the director of the Division of Oil and Public Safety may adopt, and that subsection names no octane floor for the word Regular. Subsection (6) of the same section is the grade-name provision: it bars selling a product under a grade name unless the rating or grade the name indicates “meets the applicable requirements of ASTM,” and it requires the AKI to be no less than the AKI posted on the dispenser or certified on the invoice. It states no number either. Which Colorado text fixes the number 85 is not established by the sources examined here. General Motors’ proposal to end the practice went to the regional weights and measures associations in 2012, reached the national committee in 2013 and was withdrawn at the 2016 Interim Meeting.

The same handbook table lowers the Midgrade floor from 89 to 87 and the Premium floor from 91 to 90 at altitude, and it lists no Economy grade there at all. The rule is a floor on names, not an order to sell 85, so what a station sells is a market decision. A 2013 ExxonMobil statement entered in the conference record said the majority of gasoline sold in the mountain states was Regular 85 or 85.5. In the week of November 2, 2026, EIA prices put Colorado midgrade 39.3 cents above Colorado regular, which is $5.90 more on a 15 gallon fill-up. In 2012 the same gap was 11.5 cents. What a particular engine needs at a mountain pump is a question for its owner’s manual, which the table of which grade to buy in the premium versus regular price-gap guide summarizes.

What the handbook table lets a seller call Regular

Handbook 130 is the model set of weights and measures laws and regulations that the National Conference on Weights and Measures (NCWM) maintains and the National Institute of Standards and Technology (NIST) publishes. The conference adopted its Uniform Fuels and Automotive Lubricants Regulation in 1995. States take the model up in their own legislation and rules, so each state’s text controls the stations in that state. Section 3.2.3 of the 2026 edition is the provision that governs grade names. It reads, in full: “It is prohibited to use specific terms to describe a grade of gasoline or gasoline-oxygenate blend unless it meets the minimum antiknock index requirement shown in Table 1. Minimum Antiknock Index Requirements.”

That table has two columns, one headed “ASTM D4814 Altitude Reduction Areas IV and V” and one headed “All Other ASTM D4814 Areas.” ASTM D4814 is the industry specification for automotive spark-ignition engine fuel, and section 2.1.1 of the handbook requires gasoline to meet the latest version of it. The first table below restates the handbook’s cells in the 2026 edition.

Two features of the table are easy to miss. First, it names no pump label. An 87 index gasoline clears both Regular floors, so it can carry the name Regular anywhere, and it can carry the name Midgrade only in an altitude area, where the Midgrade floor is 87. The same fuel can therefore sit under a different name on either side of an area boundary. Second, Economy has no entry in the altitude column. The 86 floor for that term exists only outside the altitude areas.

The handbook’s definition of the antiknock index, in section 1.2, is: “The arithmetic average of the Research Octane Number (RON) and Motor Octane Number (MON): AKI = (RON+MON)/2. This value is called by a variety of names, in addition to antiknock index, including: octane rating, posted octane, (R+M)/2 octane.” The EIA’s explainer on octane adds how the two numbers are measured: the research number is found under engine idle conditions with a low air temperature and slow engine speed, and the motor number under more stressful conditions of higher air temperature and engine speed. The large number on the yellow pump label, the EIA adds, is the minimum octane rating.

The floor, the motor octane limit and the half point

Two neighboring provisions shape what an 85 index pump has to deliver. Section 2.1.3 reads: “The AKI shall not be less than the AKI posted on the product dispenser or as certified on the invoice, bill of lading, shipping paper, or other documentation;” The posted number is therefore a minimum the fuel has to reach and not a target. Section 2.1.4 reads: “The minimum motor octane number shall not be less than 82 for gasoline with an AKI of 87 or greater;” The motor octane floor of 82 is tied to gasoline at 87 and above, so an 85 index gasoline falls outside the text of that sentence. Motor octane appears in the regulation’s text only in the definitions at sections 1.2 and 1.44 and in section 2.1.4; no other section sets a motor number for any grade.

That qualifier has a history. General Motors’ 2013 proposal struck the words “for gasoline with an AKI of 87 or greater” from section 2.1.4, which would have applied the 82 floor to all gasoline, and it added a sentence to section 2.1.3 setting an index floor of 87. A 2011 proposal from BP, item 237-3, went the other way and asked to delete the motor floor. Its stated reasons were that the 82 minimum is not an ASTM D4814 specification, that the FTC rule has no motor minimum, that neither the Kinder Morgan Pipeline nor the Olympic Pipeline requires a minimum motor octane specification, and that the Colonial Pipeline has no motor minimum for either reformulated or conventional blendstock. The 2013 report’s entry for the Central Weights and Measures Association (CWMA) annual meeting reads: “The submitter requested this item be withdrawn. The Committee was informed that data now supports a minimum 82 octane number and this will be placed into the language for ASTM D 4814.” The conference record shows the submitter and the fuels subcommittee agreeing to the withdrawal at the 2013 Annual Meeting.

Enforcement builds test precision in. Section 7.2.1 tells inspectors to acknowledge the antiknock reproducibility limits in Appendix X1 of ASTM D4814 when they accept or reject a sample. The posted number itself comes from the federal rule. Under 16 CFR 306.10(c), a retailer posts the rating consistent with what its supplier certified, and the half point comes from the closing sentence of that subsection, which the eCFR publishes in full as follows:

The last sentence explains the “85.5” that mountain-state marketers use. A gasoline that tests at 85.7 can be posted as 85.5 or 85 but never as 86. Section 306.5 sets how the number is found: add the research number and the motor number and divide by two. Part 306 contains no minimum octane number for gasoline. It governs how a rating is determined, certified and posted, and the floor for the name Regular comes from the handbook, which reaches a pump only through state law.

Who decides where altitude applies

Handbook 130 does not draw the map. Its altitude column is headed with the words “ASTM D4814 Altitude Reduction Areas IV and V,” and the handbook neither defines those areas nor lists the counties in them. The area designations sit in ASTM D4814 itself, a standard sold by ASTM International, and each state’s rules set the lines that its inspectors enforce. The 2013 record shows the consequence at the edges. The Alliance of Automobile Manufacturers told the conference: “States neighboring the ASTM High Altitude designated geographic areas are faced with supply, marketing, and competitive issues because of the disparity in fuel grades from high altitude states and counties, because they get their gasoline from the same limited refinery and pipeline sources.”

What the handbook adds is the incentive structure. Under the model rule, a seller inside an altitude area may sell 85 under the name Regular, and a seller outside it, drawing on the same pipeline, may not. The Alliance statement called a uniform national minimum “far preferable for consumers to a patchwork of state laws.” One regulatory official, recorded in the Western association’s comments in the 2014 report, saw the change differently: he strongly opposed the item “due to significant potential negative economic impacts in his and other Rocky Mountain States” and suggested “a ten year phase in period.”

Why altitude lowers the number: one idea, two readings

The federal sources give the reason as air pressure. The EIA glossary states the conclusion: “In general, automotive octane requirements are lower at high altitudes. Therefore, in some areas of the United States, such as the Rocky Mountain States, the octane ratings for the gasoline grades may be 2 or more octane points lower.” The Department of Energy and EPA site fueleconomy.gov supplies the history. The sale of 85 octane was originally allowed in high-elevation regions, where barometric pressure is lower, because it was cheaper and because most carbureted engines tolerated it fairly well, and the site adds that this is not true for modern gasoline engines.

The 2013 record shows where the two sides part. ExxonMobil’s statement conceded that “there have been no recent CRC studies on the subject of octane requirement needs of vehicles at higher altitudes,” and argued that “there are a number of peer reviewed scientific studies that have been published using vehicles produced during the 1990s and 2000s which confirm that an altitude adjustment for octane is still justified.” It also pointed to experience: “Consumers express their satisfaction with those grades by purchasing it overwhelmingly compared with the other octane grades available.” It continued: “We are a major marketer of these gasolines and have received no complaints from our customers about the octane over the years. And, to our knowledge, the auto manufacturers have not incurred octane-related warranty issues on vehicles from this region.”

Ford’s statement in the same record argued the opposite on warranty: “In the mid 90’s, Ford raised concerns at industry meetings and held discussions with oil companies regarding a higher rate of warranty claims that were experienced by vehicle owners in altitude regions of western states, including, the Denver area. The rate of warranty claims related to spark knock complaints in Ford trucks were significantly higher in the altitude regions as compared to areas at sea level.” Ford added that studies by the Coordinating Research Council and findings in SAE papers of the late 1980s “confirmed that Ford vehicles and others had the same octane requirement, regardless of altitude.” The Alliance wrote: “Without exception, all automakers specify the use of gasoline with a minimum octane rating of 87 AKI (Antiknock Index) in their vehicle owner’s manuals.” It added that vehicles built after 1984 “are designed, calibrated, and durability-tested to run on 87 AKI or higher fuel.” General Motors’ submission added that more vehicles were being turbocharged or supercharged, which it said eliminated the altitude intake air effect.

These are statements by parties to a rulemaking dispute, and the record shows no verdict on the facts. The CRC study that both sides were waiting for was described in 2013 as expanded, in 2014 as almost finalized and in 2015 as still being addressed, and the reports print no results from it. The record supports only a narrow summary: automakers said their engines have the same requirement at any altitude, an oil company cited peer reviewed studies and decades of sales, and the committee kept the item informational until it was withdrawn.

The rule table, with dates

The altitude column is old, and the record lets its history be dated in steps. The earliest dated piece is the federal posting rule, whose source note begins in 1979. The handbook’s table carries amendment notes for 1997 and 2018.

Two points follow from the table. The altitude column was already in the handbook when General Motors proposed to delete it, and it is still there in the 2026 edition. And the rows around it moved: the Regular Leaded line, with its 86 and 88, appears in the 2013 and 2018 prints and is absent from the 2019 reprint and the 2026 edition, both of which carry “2018” in their amendment notes. The 2018 note gives no description of the change. What the printings do show is that the Regular, Unleaded line, 85 and 87, is the same in all four.

The 2012 to 2016 attempt to end the altitude column

Item 237-2 on the conference’s Laws and Regulations (L&R) Committee agenda carried the submitter’s stated purpose: “Remove obsolete Altitude De-rating of Octane practice, establish a National Octane Baseline, and harmonize Octane Labeling from state to state.” The source was General Motors. The proposed text required an index of 87 for all gasoline, applied the motor floor of 82 to every grade, and deleted from the handbook’s Table 1 the altitude column and the Regular Leaded and Economy rows, leaving single floors of 91, 89 and 87.

The pattern in the table is deferral. The regional associations split in 2012, with the Southern and Northeastern groups recommending a vote and the Western group recommending Informational status. In the 2013 to 2015 comments recorded in the reports, the Western, Central, Northeastern and Southern associations each recommended Informational status pending the CRC study or ASTM action, so the conference left the weight on the specification body. ExxonMobil’s statement described what happened there: “a ballot to remove the discussion regarding altitude adjustments was issued last fall and addressed at the December meeting. There were a number of negatives submitted for a variety of reasons, but predominantly because there was almost no pertinent data presented that the octane requirement adjustment for altitude was invalid. It was decided to withdraw the ballot and, instead, move forward with some scientifically designed experiments to elucidate the subject.” The conference record for the 2013 Annual Meeting says the ASTM ballot failed, and the record for 2015 says the item did not pass at ASTM’s June 2015 meeting.

What each party asked for

General Motors wanted a national 87 floor. The Alliance statement, for an association that says its members account for 77 percent of car and light truck sales, asked for “a uniform, national minimum octane grade.” Ford asked the conference to review standards that “allow marketers to sell fuel with an octane rating below 87 (R+M)/2 as ‘REGULAR GRADE’.” ExxonMobil asked that the item “be maintained as informational pending ASTM action” and said a mandated increase would require refiners to invest “many millions of dollars” in unit changes. At the 2015 Western meeting another regulator said 85 had been prevalent in the marketplace for decades in the west and high altitude areas and that the proposal would have “a significant impact in the marketplace.”

Which states the record names

The model rule and the federal sources cited include no list of 85 octane states. What exists is dated testimony that names states and regions. Table 4 gives each statement with its date and speaker, and it should be read as a record of 2013 to 2015, not as a census of today’s pumps.

ExxonMobil’s list of six states spans the Rockies and the northern plains: Montana, Idaho, Wyoming, Utah and Colorado are mountain West states, and South Dakota is a Great Plains state. The Alliance statement ties South Dakota to a dispute then under way. The EIA glossary speaks of the Rocky Mountain States in general. Other states may sell the grade as well, and any of the six may have changed its rules since 2013.

How the six states stand in the handbook’s adoption survey

Section II.C of the 2026 edition tabulates, state by state, whether each state has adopted the model laws and regulations, as of November 1, 2025. Its key reads: “YES” is adopted and updated on an annual basis; “yes” is a law or regulation in force, with the NCWM standard used as the basis of adoption but from an earlier year; “yes*” is a law or regulations in force but not based on the NCWM standard; “NO” is no law or regulation; and “no” is no law or regulation, but the NCWM standard is used as a guideline. The table below gives the two fuel columns for the six states in the ExxonMobil list.

Colorado, Idaho and Wyoming therefore have fuel regulations that, by the survey’s key, are not based on the NCWM standard. Montana, South Dakota and Utah report regulations that used the NCWM standard as the basis of adoption, from an earlier year. For the first three states the handbook’s table is a national convention and not the source of the pump rule, and Colorado’s own text decides what its stations may post. The survey also warns: “Unless a state adopts the recommended regulations and subsequent amendments and revisions, there may be variation in the actual degree of adoption.” It says nothing about whether a state’s text carries the altitude column or where the area lines run.

The same testimony points to the other grades on offer. ExxonMobil said that “most of the sales in the mountain states are the Regular 85 grade,” but that ExxonMobil and other companies “do offer higher grades of 87 and 91 octane.” Under the handbook table an 87 sold in an altitude area meets the Midgrade floor and a 91 meets Premium, so a three-grade ladder of 85, 87 and 91 fits the table.

What one step up the ladder costs in Colorado

The EIA’s weekly retail series has no 85 octane line. Its grade definitions, in the gasoline glossary, are by index range: regular is “greater than or equal to 85 and less than 88,” midgrade “greater than or equal to 88 and less than or equal to 90,” and premium “greater than 90,” with a note that gasoline sales “are reported by grade in accordance with their classification at the time of sale.” A Colorado regular average can therefore include both 85 and 87 stations. The series also does not show the index of a station’s middle grade, so the gap between midgrade and regular is a stand-in for one step up the ladder, with some uncertainty about how many octane points that step spans.

In Colorado, midgrade cost 39.3 cents more than regular and premium 76.7 cents more, in a week when regular itself was $4.068. The step to midgrade was 9.7 percent of the regular price. Over the 52 weeks ending the same day, the Colorado midgrade gap averaged 39.1 cents, with a low of 35.7 and a high of 44.4, so the latest reading sits close to the year’s norm. A year earlier, in the week of October 27, 2025, the Colorado gap was 37.5 cents. Across the Rocky Mountain region the midgrade gap was 35.4 cents, and across the country 60.4 cents. The EIA’s octane explainer describes national regular as generally 87 and national midgrade as generally 89 to 90, a wider step than the 87 midgrade floor at altitude allows, which would fit the smaller Colorado gap. The price data alone do not prove that explanation.

Colorado regular was 8.5 cents below the national regular in that week, and the Rocky Mountain regular average was 6.0 cents above it. Both averages include taxes, which differ by state, and both blend whatever mix of grades each area sells, so the comparison says nothing about what an 85 octane gallon costs on its own. The state price-gap guide lays out the tax and blend differences that move one state’s average against another’s.

The history of the gap is the more useful part. From 2006 through 2012 the Colorado midgrade gap stayed between 10.8 and 11.6 cents, and the U.S. gap was within 2.3 cents of it in every one of those years. In 2013, the year of the hearing, the Colorado gap was 13.1 cents. By 2015 it was 20.6 cents, and in 2025 it was 37.3 cents, 3.2 times the 2012 level. As a share of the regular price, the step went from 3.3 percent in 2012 to 13.0 percent in 2025. The U.S. gap grew faster, from 13.8 cents in 2012 to 57.4 cents in 2025, so the national figure now exceeds Colorado’s by 20.1 cents where in 2012 it exceeded it by 2.3. The premium price-gap guide covers the national widening and the EIA study that found it did not come from refining costs.

In January 2013, when General Motors’ item was before the national committee for the first time, the Colorado midgrade gap in the four weekly readings of the month ran from 11.4 to 11.9 cents, and the premium gap from 22.7 to 23.2 cents. The ExxonMobil statement of that year remarked that “in my area of the country the differential is about 50 cents” between premium and regular. It does not say which market that describes, and the Colorado series for the month does not match it.

Dollars per tank

A price gap in cents is easier to judge as a bill. Table 8 multiplies the Colorado gaps by four example fill-up sizes. The sizes are inputs a reader can replace, and they are not claims about any vehicle’s tank.

A 15 gallon fill-up at the latest gap costs $5.90 more for midgrade, against $1.73 at the 2012 gap. A 26 gallon fill-up costs $10.22 more at the latest gap. For a year, a scale helps. The Federal Highway Administration’s 2024 tables put Colorado’s highway gasoline and gasohol use at 2,169,347 thousand gallons and its licensed drivers at 4,533,480, which is 478.5 gallons per licensed driver, the same division the gasoline use per driver by state guide makes. At that volume, paying the latest midgrade gap on every gallon would cost $188 a year more than the regular price, against $178 at the 2025 annual gap and $55 at the 2012 gap. At the latest premium gap of 76.7 cents the figure would be $367. Few drivers buy every gallon in a higher grade, and the per driver figure averages in heavy users, so these totals show the size of the price signal and not a household’s likely spend.

Why the rule is hard to move

Scale explains part of the hesitation. EIA’s refinery capacity table for the Rocky Mountain region (PAD District 4) counted 15 operable refineries on January 1, 2026, with 653,089 barrels per calendar day of atmospheric distillation capacity, which is 3.6 percent of the 18,160,493 barrels per calendar day across 130 U.S. refineries. The 2013 Alliance statement observed that neighboring states get gasoline from the same limited refinery and pipeline sources, so a change to the floor for the biggest-selling grade in the designated areas can reach border states too.

ExxonMobil’s statement put the economics in a sentence: “Octane isn’t free.” It argued that a refinery has a limited octane pool to share among its products and that removing benzene under the Mobile Source Air Toxics rule had cut octane that ethanol blending then replaced. It then warned: “If a significant increase in octane is mandated in the mountain states, we expect that refineries will have to invest many millions of dollars to modify their processing units. Those modifications take several years to plan, permit and construct, so any rapid change to regulated octanes is likely to drive supply shortage issues and pressures on the cost structure.” The automakers’ answer, in the Alliance’s words, was that consumers “need, and will continue to need nationally consistent supplies of minimum 87 AKI and higher rated octane fuels.” Both sides pointed to the CRC study and the ASTM ballots as the proper referees.

The refinery table does not show a shortage of octane-making units in the region and is not offered as proof of one. It shows scale: a region whose entire distillation capacity is under four percent of the nation’s.

Where the rule stands in November 2026

NIST’s Handbook 130 page lists the 2026 edition as the current edition and shows it last updated on January 2, 2026. The handbook’s Table 1 in that edition keeps the altitude column with the same 90, 87, 85 and not-listed cells that the 2013 print showed for those rows. The word “altitude” does not appear in the Laws and Regulations Committee’s Interim Meeting reports for 2021 through 2025 or in its agenda for the 2026 Annual Meeting. The 2017 Interim report does not use it, and the 2018 and 2019 Interim reports use it only where they reprint the table or its heading; the 2019 reprint sits inside an item on ethanol blend terms that was withdrawn and did not concern altitude. Within those L&R documents, the question that General Motors, Ford and the Alliance raised in 2013 has not come back since the 2016 withdrawal, and the 2026 Annual Meeting agenda shows no altitude item pending.

That is a statement about the conference. ASTM D4814 and each state’s rules can change on their own schedules; Colorado’s Senate Bill 26-122 (Session Laws chapter 109), signed May 5, 2026 and effective August 12, 2026, adds a paragraph (1)(b) to C.R.S. 8-20-204. It lets the director of the Division of Oil and Public Safety “adopt a rule or issue policy guidance that provides exceptions to specific requirements established in an ASTM standard” incorporated by paragraph (1)(a), “to address consumer protection, pricing, supply networks, or other factors not considered by ASTM when formulating the standard.” The act does not mention octane, and nothing examined here shows whether any such exception covers it. The record does show the allowance surviving a four year challenge by the largest automakers, with both sides citing studies the reports do not reproduce and the conference deferring to ASTM.

Related questions on this site include why seasonal blends move prices, in the summer versus winter gasoline guide; what E15 does to the grade names, in the E15 explainer; and what sits inside a pump price, in the gasoline price breakdown. The fuel types hub collects the other grade and blend questions.

Data and sources, all checked November 3, 2026: National Institute of Standards and Technology, NIST Handbook 130, current edition (2026 edition; Section IV.F, Uniform Fuels and Automotive Lubricants Regulation, sections 1.2, 1.44, 2.1.1, 2.1.3, 2.1.4, 3.2.3, 7.2.1 and Table 1). National Conference on Weights and Measures, Laws and Regulations Committee final reports: 2013 (items 237-2 and 237-3) with Appendix E (statements of January 27, 2013), 2014 and 2016, and the Interim Meeting reports for 2017 through 2019 and 2021 through 2025 with the 2026 Annual Meeting agenda on ncwm.com. Colorado General Assembly, Senate Bill 26-122 (bill page, and Session Laws chapter 109, which prints C.R.S. 8-20-204(1) as amended, and Colorado Revised Statutes 2024, title 8, section 8-20-204(6), uncertified printout on leg.colorado.gov, checked November 3, 2026). Federal Trade Commission, 16 CFR Part 306 in the eCFR (current through October 30, 2026; sections 306.5 and 306.10). U.S. Energy Information Administration: gasoline grades glossary; Gasoline explained: octane in depth; Colorado regular, midgrade and premium weekly retail prices, with the Rocky Mountain and U.S. series and the annual views (release of November 3, 2026); Number and Capacity of Petroleum Refineries, PAD District 4 (release of June 26, 2026). Federal Highway Administration, Highway Statistics 2024, Table MF-21 and Table DL-1C. Department of Energy and EPA, fueleconomy.gov, Selecting the Right Octane Fuel. ASTM D4814 and the individual state fuel rules are not reproduced here, so the area designations come only from the cited records. The L&R search covered the documents named above; the 2020 report and the fuels subcommittee reports are not among them. Gaps, ratios, shares and per tank and per year costs are this page’s arithmetic.