How much is it more fuel efficient to drive at 65 mph versus 75 mph?
What's the best speed to drive at on a highway to get optimal
fuel efficiency?

The answer to that question depends on what kind of car you're driving.  The 
optimal speed for a car depends on several factors including weight and how 
aerodynamic the car is.  However, according to www.fueleconomy.gov, gas mileage 
decreases rapidly at speeds above 60 miles per hour.  A rough estimate is that 
every 5 mph over 60 decreases your fuel efficiency by about 7%.

Here's an article that discusses the question of speed and fuel efficiency.  
One thing it mentions is that most cars' optimal fuel efficiency falls between 
40 and 60 mph.  Bigger, heavier vehicles like trucks and SUVs will hit their 
peak efficiency at a lower point, while smaller and more aerodynamic cars reach 
theirs at higher speed.

http://www.howstuffworks.com/question477.htm

Thanks!