Your dashboard's MPG readout is an estimate, and it's usually flattering. The pump tells the truth, and extracting your real gas mileage takes about thirty seconds of division. The method below, one fill-up, one odometer, one receipt, is the same one test labs and consumer publications use.
One tank is a snapshot. Pump shutoff points differ slightly, and a week of city errands isn't a week of commuting. Average three to five tanks before drawing conclusions, especially before blaming the car or patting yourself on the back for new driving habits.
Trip odometer reads 312 miles. The pump takes 11.9 gallons at $3.45 a gallon. Your MPG: 312 ÷ 11.9 = 26.2 mpg.
From there, three useful derivations. This fill-up cost 11.9 × $3.45 = $41.06. Fuel costs $3.45 ÷ 26.2 = 13.2 cents per mile. And at 13,500 miles a year, the car burns 13,500 ÷ 26.2 = 515 gallons, an annual fuel bill of about $1,778.
Enter miles, gallons, and price. Get MPG, cost per mile, yearly fuel cost, and L/100km.
MPG Calculator →Most cars estimate fuel consumption from injector pulse timing rather than measuring fuel flow directly, and the error usually lands on the sunny side, a few percent high in most comparisons against pump math. Over a year that gap is worth real money in your mental budget: a dash claiming 28.5 mpg while the pump says 26.2 overstates your efficiency by 9%. Measure before you celebrate.
The trap in MPG is that it's a ratio, and savings per extra point shrink as you climb. The honest currency is gallons per 100 miles.
| Move | Gallons per 100 mi | Saved per year at 12k mi, $3.50 |
|---|---|---|
| 15 → 20 mpg | 6.67 → 5.00 | $700 |
| 20 → 25 mpg | 5.00 → 4.00 | $420 |
| 25 → 30 mpg | 4.00 → 3.33 | $280 |
| 30 → 40 mpg | 3.33 → 2.50 | $350 |
| 40 → 50 mpg | 2.50 → 2.00 | $210 |
Upgrading a 16 mpg truck to a 20 mpg truck saves more fuel than swapping a 35 mpg sedan for a Prius, which is why the biggest efficiency wins live at the bottom of the scale. If you're choosing between two cars, divide miles by each MPG and price the gallons.
Divide 235.215 by the MPG: 26.2 mpg = 235.215 ÷ 26.2 = 8.98 L/100km. The constant comes from the unit conversions, 3.785 liters per gallon, 1.609 kilometers per mile, and the scale flips: lower is better. Cars sold in Canada and Europe quote L/100km, so this conversion is how you compare a Canadian spec sheet to a US window sticker.
Tracking MPG over a few tanks is also free diagnostics: a sudden unexplained drop is often a dragging brake, a bad oxygen sensor, or a tire losing pressure, all cheaper to fix early.
At a full tank, reset the trip odometer. Drive until the next fill-up, then refill completely and keep the receipt. MPG is trip miles divided by gallons on the receipt: 312 miles on 11.9 gallons is 26.2 mpg. Average three or more tanks for a trustworthy number.
Usually a little optimistic. Instant-read systems estimate fuel flow rather than measuring it, and most read 2% to 5% high compared with pump math. The pump is the measuring instrument you already own, which is why the full-to-full method beats trusting the dash.
It depends where on the scale you are, because gallons per mile is what bills you. At $3.50 a gallon and 12,000 miles a year, moving from 15 to 20 mpg saves $700, from 20 to 25 saves $420, from 25 to 30 saves $280, and from 40 to 50 saves $210. Each extra mpg is worth less at the top, since each saves fewer gallons.