Thanks guys. I actually understand pretty well how it affects aircraft. I know in theory it should have some effect on a car, but I also know that its really hard to test because of the aforementioned problems such as more hills in high elevation places. Just wondering if the difference was every noticeable, or if it was small enough that it got swamped in other effects. Sounds like at least some of you have noticed though. Gonna take everyone's word for it for now, because I don't feel like A-B-A testing the whole thing by making a few runs in Denver
BTW, why the suggestions that some diesels would be different? Only difference I can see is that most of them are turbocharged, so you wouldn't lose power at altitude. Still, less drag=less power required to maintain a given speed.