Your real MPG from fill-ups, cost per mile, and what any trip will cost
Your car's real MPG is the number behind every fuel decision — trip budgets, commute costs, whether the thirsty truck is worth it — and it's rarely the window-sticker figure or the optimistic dash display. The honest method takes ten seconds at the pump: miles since last fill-up ÷ gallons to refill. This calculator turns that into cost per mile, trip costs, annual spend, and the savings a more efficient vehicle would actually deliver.
Onboard computers estimate fuel flow; studies consistently find them 5–8% optimistic. The tank-to-tank method measures reality: fill to auto-shutoff, zero the trip odometer, drive normally, refill to shutoff at the same pump speed — miles ÷ gallons. Three consecutive tanks averages out fill variance. (EV drivers: the equivalent is mi/kWh — the EV vs Gas tool speaks that language.)
| Factor | Impact |
|---|---|
| Speed above 60 mph | −7% per 5 mph (aerodynamic drag squares with speed) — the 75-vs-65 choice costs ~15% |
| Aggressive acceleration/braking | −15–30% city; the biggest driver-controlled factor |
| Underinflated tires | −0.2% per PSI down (see the Tire Pressure guide) |
| Roof racks/cargo boxes | −10–25% at highway speed |
| Winter (cold engine, winter blend, idling) | −10–20% |
| AC vs windows | ~−3% either way; wash |
At 29 MPG and $3.20/gal, fuel costs 11¢/mile — so a 500-mile trip is $55, a 12,000-mile year is $1,325, and a friend's "gas money" for a 60-mile favor is $6.60 (plus wear — the IRS's 70¢/mile approximates the true all-in cost). The +10 MPG card reframes vehicle shopping: for a typical driver, the difference between 22 and 32 MPG is ~$550/yr — real, but smaller than one depreciation year on a new-car swap "to save gas" (run the Depreciation tool before that logic wins).
EPA figures come from lab cycles; real traffic, speed, weather and cargo typically land 10-20% under the sticker (hybrids gap most in winter). Your tank-to-tank number is the truth for budgeting.
If your car SPECIFIES premium, use it (knock-limited engines lose power and efficiency on regular). If it merely 'recommends,' regular usually wins on cost — a 20-30¢ spread rarely buys 7% more efficiency. Check the fuel-door sticker wording.
Independent testing says no, consistently — the FTC has pursued dozens of such claims. Tire pressure, smooth driving and speed discipline deliver more than any bottle.
Most vehicles peak at 45-60 mph in top gear; above that, drag dominates. The practical rule: on the highway, every 5 mph over 65 costs ~7% — cruise control at the limit is the free efficiency tech.
Total trip miles ÷ total gallons across all fill-ups — same method, summed. Highway trips usually read 15-25% above your commuting MPG; use the trip figure for trip budgets.
Idling burns 0.2-0.5 gal/hour getting 0 MPG — a 10-minute daily warm-up is ~$60-100/yr. Modern engines want 30 seconds, then gentle driving.
Yes — every figure computes locally in your browser.
Measure three tanks, learn your cost per mile, and every fuel question — trips, commutes, the truck-vs-sedan debate — becomes arithmetic. The pump ritual takes ten seconds and outperforms the dashboard's flattery.