We’ve all heard the tips on how to save gas (“slow down,” “avoid hard acceleration,” etc.) Here are a few more with percentage of estimated fuel savings, and the tailgate-down myth. Your vehicle uses 10 to 15 percent more fuel for every 5 mph above 60 mph. Properly tuned vehicles improve gas mileage up to 6 percent. Having properly inflated tires increases fuel efficiency by 3 to 5 percent. Wind resistance from a loaded roof rack can reduce fuel efficiency by 5 percent. Heavy loads: An extra 100 pounds reduces fuel efficiency by 2 percent. Idling for more than 30 seconds uses more fuel than stopping and restarting the engine. The A/C myth: Modern cars are designed to use less fuel with the windows up and the air conditioning on than with the A/C off and windows down. A loose or missing gas cap allows fuel to evaporate. Also, when the pump automatically clicks off, stop fueling. Any "extra" gas you can get will probably seep out. A dirty air filter or worn-out spark plugs can cost you nearly two miles per gallon. A faulty oxygen sensor, your gas mileage may be decreased by as much as 40 percent. Tailgate myth: It's an urban myth that driving a pickup truck with the tailgate down will decrease wind resistance. A study published by the National Research Council of Canada says keeping it down (or using a mesh tailgate in its place) actually has the opposite effect. When it's up, a bubble of air forms in the truck bed, and air flows more smoothly over and off the truck, without as much drag. A tonneau cover works best.