Before a road trip, it helps to know what the gas will cost. With your distance, your vehicle's fuel economy, and the price at the pump, the math is quick.
The calculation
Divide the trip distance by your miles-per-gallon to get the gallons you'll burn, then multiply by the price per gallon. The cost-per-mile figure is handy for comparing vehicles or estimating longer trips. A more fuel-efficient vehicle lowers both numbers directly.
A worked example
A 300-mile trip in a car that gets 30 MPG uses 10 gallons; at $3.50 a gallon that's $35, or about 12 cents a mile. Drop to a 20-MPG vehicle and the same trip costs $52.50 — 50% more — while a 40-MPG car brings it down to about $26. Fuel economy moves the total more than modest swings in the pump price do.
Estimating a longer trip or a year of driving
The cost-per-mile figure makes bigger estimates easy: multiply it by your expected annual mileage to ballpark a year of fuel. For a round trip, remember to enter the full there-and-back distance.
If you're weighing two vehicles, comparing their cost per mile is a cleaner apples-to-apples number than MPG alone, and it's the same figure you'd use to check whether a longer but faster highway route actually costs more in gas.