A study in the uk in the 90s stated the most economic speed to drive a car is 57 mph. However as you say its more car specific... funny thing is the least efficent speeds are 30 -40 mph.. according to the same study.Does you car have an onboard computer ??? My sable had yukon do.... it tells you the estimated fuel consumption at the speed you are driving... the sable seems to be greatest at 50 the Yukon at 55-60 . The yukon drinks at 70+ the sable drops to 28 mpg at 70......