MPG Calculator
Your real gas mileage from miles driven and gallons used.
Input sheet
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Your car's window-sticker MPG is a lab estimate; your real mileage depends on how and where you actually drive. Tracking it from a single tank tells you your true fuel economy and flags problems early.
How it works
Miles driven divided by gallons used since the last fill-up.
The method is simple: fill the tank, reset the trip odometer, drive normally, then refill and note the miles and gallons. Miles divided by gallons is your real MPG for that tank. Because conditions vary, a few tanks averaged together give a more reliable number than any single fill-up.
The result also converts to L/100km and km/L for international comparison. Note that L/100km is inverse — a lower number is better — which is why it's the standard in much of the world for judging efficiency.
MPG = miles driven ÷ gallons used. L/100km = 235.215 ÷ MPG. Km per liter = MPG × 0.425.
Worked examples
You drove 320 miles and the tank took 11.5 gallons to refill. → 27.8 MPG (about 8.5 L/100km, or 11.8 km/L).
320 ÷ 11.5 = 27.8 MPG. Converting: 235.215 ÷ 27.8 ≈ 8.5 L/100km.
A highway tank of 400 miles on 14 gallons. → 28.6 MPG.
400 ÷ 14 = 28.6 MPG — slightly higher than the city tank above, which is typical since steady highway speeds are more efficient.
Tips & gotchas
- Always fill to the same click-off point both times — topping off unevenly throws the gallons figure off and skews the result.
- A sudden drop in MPG with no change in driving can signal under-inflated tires, a dirty air filter, or a failing oxygen sensor — treat it as a diagnostic clue.
- Average at least three tanks for a number you can trust; one tank can be distorted by a single long highway trip or a week of short cold-start commutes.
- City driving, cold weather, roof racks, and aggressive acceleration all cut MPG noticeably — compare like-for-like tanks when judging a change.
FAQ
Why is my measured MPG lower than the EPA rating?
EPA ratings come from controlled lab cycles. Real-world short trips, cold starts, idling, highway speeds above 65 mph, and cargo all reduce economy, so most drivers see numbers a few MPG below the sticker.
Can I trust the car's built-in MPG display?
Onboard displays estimate fuel flow and are often optimistic by 5–10%. A hand-calculated number from actual gallons pumped is the ground truth — use it to check and recalibrate the dashboard reading.
Should I measure city and highway separately?
Yes, if you want to understand your driving. Keep one trip's worth of mostly-city miles and one of mostly-highway miles and calculate each — the spread tells you where your fuel actually goes.
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