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.