Today, 35 m.p.h. is no longer the best speed for autos with their sleek designs and advanced transmissions. Newer vehicles generally get the highest gas mileage somewhere between 45 and 55 m.p.h., says David L. Greene of the National Transportation Research Center at Oak Ridge National Laboratory in Knoxville, Tenn.My guess is that the differences would be even more significant for older cars. I think this is the referenced Consumer Reports article.
The main force reducing mileage is air drag, says Dr. Greene. The faster you go, the greater the drag. Drag forces increase exponentially, so doubling your speed from 40 to 80 increases drag fourfold.
It makes a huge difference, for at 80 m.p.h. your car pushes against wind with the force of a hurricane.
Consumer Reports tested the effect of higher speeds on gas mileage. David Champion, director of auto testing, found that boosting the highway speed of a 2006 Toyota Camry cut gasoline mileage dramatically:
•55 m.p.h. – 40.3 miles per gallon
•65 m.p.h. – 34.9 miles per gallon
•75 m.p.h. – 29.8 miles per gallon
On a hypothetical 1,900-mile round trip from New York City to Disney World in Florida, the Camry would use 47 gallons of gas at 55 m.p.h.. But at 75 m.p.h., it would burn nearly 64 gallons – a $70 difference.
PS - Not that the President would ask you to make such a "partiotic" decision as to save gas.