Just going 65 mph can cut gas efficiency more than 10%. Also, using cruise control helps.
MAINTAIN YOUR TIRES
Underinflated tires can increase fuel consumption by 6%.
KEEP THE ENGINE TUNED
An untuned engine can cut mileage by 10% to 20%.
CHANGE THE AIR FILTER
A clogged air filter can drop fuel economy by up to 10%.
SKIMP ON THE AC
Air conditioning can cut mileage per gallon by as much as 20%.
Data: Energy Dept., Environmental Protection Agency