While driving 80 does save time, the reverse of it it reduces a car's mpg. Vehicle wind resistance for the majority of cars increases the faster you go, decreasing mpg. 

The increase is actually exponential, meaning wind resistance rises much more steeply between 70 and 80 mph than it does between 50 and 60. "

This site lets you compute how much your mpg will drop by going faster. For my 30 mpg car, driving 80 mph reduces mpg by 28%. Meaning I'm buying more gas to go the same distance at 80 mph versus 65 mph. Not worth it.

I'll be the car on the right going 65 mph while everyone else blows by me wasting gas. Aloha.

http://www.mpgforspeed.com/