Yes, cars use slightly more fuel at higher speeds however:
A) According to the EPA cars and trucks make up less than 2.3% of the worldwide man-made "greenhouse" gases. Hardly a siginificant amount to worry about.
B) Cras usa a a lot more fuel in stop in go traffic than on highways.
C) Many modern cars get about the same fuel mileage at 70 than they do at 55.
and finally
D) If speed doesn't increase with higher limits
how can higher speed limits use more fuel?