It would appear that in the US of A, most fuel is generally sold as Regular, Plus (or Midgrade), and Premium, with the actual specification being hidden in small print.

My car requires 87, which goes as Regular at sea-level in Houston or San Jose, but the Regular in Denver is 85. Without getting into too much details, it would appear that although modern cars would still run OK with 85 in Denver, the experts do suggest that such use is no longer optimal. Not to mention the possibility of filling up at an elevation only to continue the trip towards the sea level.

As such, when doing cross-country travel, tools like GasBuddy.com may be misleading in advertising you suboptimal fuel. E.g., a combination of fuel tax jurisdiction and fuel grade may amount for significant differences between nearby stations.

Is there a solution? Automated would be best, but a map of mappings between "REG" et al and "85"/ "87" et al would be great as well.

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    As far as I'm aware, federal law requires stations display the actual octane rating on a sticker at the pump, as in an actual number rather than a word. That's not helpful to you if you're trying to find a station in advance to compare prices, but you should always know what you're getting before you buy. – Zach Lipton Jul 21 '17 at 22:00
  • @ZachLipton, yes, exactly -- the numbers are always displayed once you're at the pump, but GasBuddy isn't regulated. :-) – cnst Jul 21 '17 at 22:05
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    There's a partial list at en.wikipedia.org/wiki/U.S._State_Fuel_Octane_Standards – Nate Eldredge Jul 21 '17 at 23:40
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    My experience is that most states start at 87 and those that offer 85, usually also have higher octane options. Is mapping out your gas stations in advamce that important? – user13044 Jul 22 '17 at 6:03
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    Does it really vary by elevation within states? Colorado has elevations from 3300 to 14000 feet but I believe 85 octane is sold throughout the state. Conversely, California varies from -282 to over 14000 but I have never seen anything lower than 87 there. – Nate Eldredge Jul 22 '17 at 19:03

No such map currently exists as of 2017. Your best bet is therefore to find a cheap gas station through GasBuddy and then hope their higher-octane fuels are also reasonably priced.

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