So a few weeks ago on the show a caller from Denver asked about using a lower fuel octane at altitude and the answer was there is no harm since the compression in the chamber is lower with the thinner air you can use a lower octane. So that got me to thinking if using a higher octane fuel than recommended is actually worse for the car. You usually read that there is no benefit to using a higher octane, but it is actually worse? If the compression in the chamber isn't high enough are you just not exploding all the gas and getting worse mileage? I live in Denver as well and my car recommends 91, since the next lowest is 87 I have just kept putting 91 in the car but after the call got to thinking that I may actually be losing performance by using 91 since I am at altitude and my cars compression is effectively lower. Could this be the case?