but not too slow, you will very poor mileage below 20 mph, I am sure. The optimum speed will change from car to car. Around 40 mph is probably a good number for most cars. In city trafic it is better to coast to a red light than to stay on the accelerator and then brake. If you can time the lights so much the better. I agree that there are no magical sweet spots at high speed. An no formulas either.

JL