Gas mileage — miles per gallon (MPG) — tells you how far your car travels on a gallon of fuel. Tracking it is the simplest way to spot a maintenance problem or compare vehicles, and it's the number that drives every other fuel calculation.
How to measure your real MPG
Fill the tank, reset the trip odometer, then at your next fill-up divide the miles driven by the gallons it took to refill. Doing this over a few tanks smooths out the effect of traffic and terrain.
The metric equivalents help when comparing with cars rated in liters: L/100km (lower is better) and kilometers per liter (higher is better) describe the same efficiency a different way.
Why MPG matters more when gas is expensive
A few MPG makes a bigger dollar difference at higher pump prices. Keeping tires properly inflated, easing off hard acceleration, and removing extra weight can each nudge your mileage up.
A worked example
Drive 300 miles on 10 gallons and your gas mileage is 30 MPG (300 ÷ 10) — about 7.84 L/100km or 12.75 km per liter. Tracking this across several tanks is far more telling than any single fill-up: a sudden, sustained drop can flag under-inflated tires, a dragging brake, or a tune-up that's overdue.
Reading the metric equivalents
MPG and km/L both rise as efficiency improves, but L/100km moves the opposite way — it measures fuel used per distance rather than distance per fuel, so lower is better. That reversal trips people up when comparing a car rated in one system against a car rated in another.
A 30 MPG car and a 7.84 L/100km car are the same efficiency described two ways, so convert both to a single unit before deciding which is thriftier.