To Save Gas: Drive Sensibly
Aggressive driving (speeding, rapid acceleration and braking) wastes gas. It can lower your gas mileage by 33 percent at highway speeds and by 5 percent around town. Sensible driving is also safer for you and others, so you may save more than gas money.
Fuel Economy Benefit:
5-33%
Equivalent Gasoline Savings:
$0.15-$1.01/gallon

Observe the Speed Limit:
Graph showing MPG VS speed MPG decreases rapidly at speeds above 60 mphWhile each vehicle reaches its optimal fuel economy at a different speed (or range of speeds), gas mileage usually decreases rapidly at speeds above 60 mph.

As a rule of thumb, you can assume that each 5 mph you drive over 60 mph is like paying an additional $0.21 per gallon for gas.
Observing the speed limit is also safer.
Fuel Economy Benefit:
7-23
Equivalent Gasoline Savings:
$0.21-$0.71/gallon

Remove Excess Weight:
Avoid keeping unnecessary items in your vehicle, especially heavy ones. An extra 100 pounds in your vehicle could reduce your MPG by up to 2%. The reduction is based on the percentage of extra weight relative to the vehicle's weight and affects smaller vehicles more than larger ones.
Fuel Economy Benefit:
1-2%/100 lbs
Equivalent Gasoline Savings:
$0.03-$0.06/gallon

Avoid Excessive Idling:
Idling gets 0 miles per gallon. Cars with larger engines typically waste more gas at idle than do cars with smaller engines.

Use Cruise Control:
Using cruise control on the highway helps you maintain a constant speed and, in most cases, will save gas.

Use Overdrive Gears:
When you use overdrive gearing, your car's engine speed goes down. This saves gas and reduces engine wear.

Source:
Estimates for fuel savings from sensible driving are based on studies and literature reviews performed by Energy and Environmental Analysis, Inc., Washington, DC.