Work out the fuel economy your car really gets, not the sticker number: enter the odometer readings (or miles driven) between two fill-ups and the gallons the tank took, and see miles per gallon, L/100 km, cost per mile, what fuel costs you per year and how much a more efficient car would save.
Fuel economy
28.8 MPG
8.2 L/100 km · 12.2 km/L
Cost per mile
11.3¢
this tank: $37.05 for 328 mi
Fuel per year
$1,525
469 gallons
vs 40 MPG car
−$428
per year
328 miles ÷ 11.4 gallons = 28.77 MPG. Range on a full tank ≈ tank size × MPG (a 14-gallon tank: 403 miles).
📌 Fill the tank to the same point each time (first click-off) and reset the trip odometer; average three or more tanks for a reliable number. Conversions: L/100 km = 235.2 ÷ MPG; UK (imperial) MPG = US MPG × 1.2.
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Yearly fuel cost by MPG
13,500 miles a year (the US average) at $3.25 per gallon. Fuel use is proportional to 1 ÷ MPG, so each step up saves less than the one before.
Use the same pump and stop at the first automatic click-off both times; topping off adds gallons that distort the result. Track at least three consecutive tanks — 900–1,000 miles — and average them. Check tire pressure (each 1 psi under costs about 0.2 % in MPG), remove roof boxes when not in use, and remember that winter blends and cold weather typically cut fuel economy by 10–20 %. The car’s own trip computer usually reads 3–8 % optimistic, which is exactly why a fill-up calculation is worth doing.
Frequently asked questions
Fill the tank, note the odometer (or reset the trip meter), drive normally, fill up again and divide the miles driven by the gallons the second fill took. 328 miles on 11.4 gallons is 28.8 MPG. The first fill sets the baseline; the gallons on the second fill are what you actually burned. Repeat over several tanks and average — a single tank can be off by 10 % from how full the pump clicked off.
The window-sticker figures come from standardized lab cycles; real driving adds cold starts, short trips, air conditioning, roof racks, under-inflated tires, speeds above 65 mph and traffic. Most drivers land within 10–15 % of the combined rating, and hybrids often beat the sticker in city driving while trucks and sports cars usually fall short of it.
The US new-vehicle average is about 27–28 MPG combined. Compact sedans get 32–40, hybrids 45–55, mid-size SUVs 22–28, full-size pickups 17–22. Anything above 35 MPG is efficient; below 20 you are paying roughly twice as much per mile as a compact — the comparison box above shows exactly what that costs a year.
At 13,500 miles a year and $3.25 a gallon, a 25 MPG car costs $1,755 in fuel and a 30 MPG car $1,463 — a $292 gap. Going from 20 to 25 MPG saves more ($439) than going from 30 to 35 ($209), because fuel use is proportional to 1 ÷ MPG. That is why gallons-per-100-miles (or L/100 km) is the fairer way to compare.
Divide 235.2 by the MPG: 28.8 MPG is 8.2 L/100 km, 40 MPG is 5.9 L/100 km. The other direction is the same division. British (imperial) gallons are 20 % larger, so a UK figure of 40 MPG equals 33 US MPG.