Vehicle speed is another very important factor affecting fuel economy. The rule of thumb to remember is that fuel economy will change approximately 0.1 mpg for every 1 mph speed change above 55 mph. In other words, decreasing vehicle speed from 70 mph to 65 mph can improve fuel mileage by approximately 0.5 mile per gallon. The actual fuel mileage improvement depends on the tractor-trailer aerodynamics, ambient temperature, gross weight, and tire type. As speed increases, tractors with poor aerodynamics will experience greater fuel economy loses than vehicles with better aerodynamics.