This formula is my best "human" approximation of my observed experiences. Don't worry the fuel economy would be far superior in Dallas where the land is flatter, hotter and less... well... New Jersey driving combat zone. Although, I cannot say that my experiences driving in Dallas were wonderful. I have a 2001 and I have been unable to get higher than low forties mpg at sustained at 75. It takes a lot of prediction work in order not to accelerate or decelerate very much, that is how you get better milage. I guess my formula was off a bit, I never get below 37 mpg. hmm.
Aron