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.