how much does heat effect fuel mileage and weight in a car like this? had about 650 lbs in passengers (3 people), and it was apprx 90* out today cruising on the interstate. I was disappointed in the fuel mileage at 70mph, we were only seeing about 32mpg or so. Granted most of the driving was on slight grades for long distances. When I dropped the speed to 68mph I saw a 3 or so mpg gain 65 ish and below was better by another 1-2mpg it was a little windy as well now that I think back on the trip.