Octane

[schnowz]

Ped I'd forgotten the European rating is different to the US.
 which I believe gives a higher number in Europe for the same gas.
from Wiki
 
On the East coast here most gasoline has 10% alcohol which gives lower gas mileage. It could be that Colorado at that time had no alcohol.
"In most countries, including Australia and all of those in Europe[citation needed], the "headline" octane rating shown on the pump is the RON, but in Canada, the United States, Brazil, and some other countries, the headline number is the average of the RON and the MON, called the Anti-Knock Index (AKI, and often written on pumps as (R+M)/2). It may also sometimes be called the Pump Octane Number (PON).