Quick answer: To calculate MPG, divide the miles you drove by the gallons of fuel you used: MPG = miles ÷ gallons. If you drove 312 miles and refilled 11.2 gallons, your car gets about 27.9 MPG.
Knowing your real fuel economy lets you budget for trips, spot engine or tire problems early, and compare cars honestly. The number on the sticker is a lab estimate. Your own MPG is the one that matters.
How to calculate MPG (the fill-up method)
- Fill the tank completely at the pump and stop at the first click. Don’t top it off.
- Reset your trip meter to zero (or write down the odometer reading).
- Drive normally until the tank is at least half empty. A longer stretch gives a more accurate answer.
- Fill the tank completely again and note how many gallons the pump added.
- Divide miles by gallons. Example: 312 ÷ 11.2 = 27.86, or about 27.9 MPG.
For the most reliable result, repeat this over three or four tanks and average them. One fill-up can be thrown off by pump differences, a hot day or a stretch of heavy traffic.
What to do with your MPG number
Once you know your MPG, you can turn it into money. Our guide to calculating fuel cost for a road trip shows how to budget a journey, and gas cost per mile explains how to compare your daily drive with other options.
Converting MPG to L/100 km
Many countries measure fuel economy as liters per 100 kilometers (lower is better). The conversion is 235.215 ÷ MPG for US gallons.
- 27.9 MPG → 235.215 ÷ 27.9 = 8.4 L/100 km
- 35 MPG → 6.7 L/100 km
- 20 MPG → 11.8 L/100 km
Watch out: a UK (imperial) gallon is bigger than a US gallon (4.55 liters versus 3.79), so a car’s UK MPG looks about 20% higher than its US MPG. Make sure you’re comparing the same gallon.
Why your MPG may be lower than expected
- Under-inflated tires increase rolling resistance.
- Fast driving and hard braking use noticeably more fuel than steady speeds.
- Short trips and cold weather keep the engine less efficient.
- Extra weight and roof racks add drag and load.
- Skipped maintenance such as old spark plugs or a dirty air filter can hurt economy.
Skip the math
Enter your distance, MPG and fuel price into the Fuel Cost Calculator and it works out the trip cost for you.
Frequently asked questions
What is a good MPG?
It depends on the vehicle type. Many modern gasoline cars land somewhere in the high 20s to mid 30s in mixed driving, while hybrids go higher and large trucks and SUVs go lower. Compare your result with your own car’s rated figure rather than a single universal number.
How do I calculate MPG without a trip meter?
Write down the odometer reading at each fill-up. Subtract the earlier reading from the later one to get miles, then divide by the gallons you added at the second fill-up.
Is my car’s trip computer accurate?
It’s usually close but can be off by a few percent. The fill-up method is more reliable because it uses real fuel and real distance.
How do I calculate MPG for an electric car?
EV efficiency is usually shown as kWh per 100 miles or miles per kWh. Divide miles driven by the kWh used (from your charging records) to get miles per kWh.
How many gallons will I need for a trip?
Divide the trip distance by your MPG. A 300-mile trip at 30 MPG needs 10 gallons.
Last updated: October 2026. Fuel prices in examples are illustrative.
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