Have you considered the difference in gas?

I don't think Vegas is really at too high of elevation.

The worst gas I ever had was at a tank in Denver in the early 90's. Driving an 88 Nova at the time, but heading west out of Denver, I could not even go 65 miles an hour unless there was a slight downgrade. Couldn't even maintain 65 on the level.

Gas mileage was terrible too, and when I got it down to 1/2 tank, filled up with new gas and things got much better.

For what it's worth, on the trip to Colorado, even though I had a steady climb accross Nebraska, I had the best gas mileage ever in that car.