MPG Calculator
Find your real-world miles per gallon from a single fill-up, convert it to any unit used worldwide, and see how much every mile is actually costing you. Enter odometer readings or a simple trip distance, add a fuel price, and get the full picture in seconds.
Formula
Worked example
300 miles on 10 US gallons: 30 MPG (7.84 L/100km, 12.75 km/L). At $3.50/gal that is $0.117 per mile and $35 total.
How the MPG calculation works
Miles per gallon is simply the distance you drove divided by the fuel you used over that exact distance. The most accurate method is the fill-to-fill technique: brim the tank, reset the trip odometer, drive normally, then brim the tank again at the same pump position. The gallons shown on the second receipt and the miles on the trip counter go straight into this calculator. The odometer-reading mode lets you enter the start and end readings instead of the trip distance, which is useful if you forgot to reset the counter. All three output units (MPG, L/100km, and km/L) describe the same underlying ratio; L/100km is the inverse of MPG scaled for kilometres and litres, while km/L is the metric equivalent of MPG.
Gallon types and why they matter
A US liquid gallon equals 3.785 litres. A UK imperial gallon equals 4.546 litres, roughly 20% more. This means a car with a true fuel economy of 30 MPG (US) would show about 36 MPG if the driver mistakenly applied UK gallons to the same fill. When comparing your result to a posted EPA rating or a manufacturer specification, confirm that both figures use the same gallon standard. The calculator lets you choose US gallons, UK gallons, or litres, and it normalises everything to US gallons internally before computing MPG.
Cost per mile and total fuel spend
Enable the fuel cost toggle and enter the price per gallon (or per litre) at the pump to unlock two extra outputs: cost per mile and cost per kilometre. Cost per mile is simply the fuel price divided by your MPG. At 30 MPG and $3.50 per US gallon, every mile costs $0.117. Over a year of 15,000 miles, that is $1,750 in fuel alone. Knowing your cost-per-mile lets you compare the true running cost of different vehicles or estimate how much a proposed route will cost before you drive it.
What affects real-world fuel economy
Driving style is the single largest variable: aggressive acceleration and hard braking can reduce economy by 15-30% compared with smooth, steady progress. Aerodynamic drag grows with the square of speed, so every 5 mph above 60 mph typically costs around 7-14% in MPG. Tire pressure matters too: underinflation by 10 psi can cut economy by roughly 0.5-3.3%. Cold engines and short trips are expensive because modern engines run in a fuel-rich open-loop mode until they reach normal operating temperature, typically around 195 degrees Fahrenheit. Air conditioning can add 5-25% to fuel use depending on ambient temperature and the size of the AC system. Real-world fuel economy is almost always 10-20% below the EPA label estimate, which is generated under controlled laboratory conditions.
Typical MPG by vehicle class (US EPA combined estimates)
| Vehicle class | Typical MPG (combined) | Efficiency rating |
|---|---|---|
| Compact hybrid / EV-range extended | 50-70+ | Excellent |
| Compact / midsize sedan | 30-40 | Good |
| Full-size sedan / midsize crossover | 25-32 | Average |
| Midsize SUV / minivan | 20-28 | Below average |
| Full-size SUV / light truck | 15-22 | Below average |
| Heavy-duty pickup / large SUV | 10-18 | Poor |
Real-world results are typically 10-20% below these EPA ratings. Source: fueleconomy.gov.
Frequently asked questions
What is the difference between a US gallon and a UK (imperial) gallon?
A US liquid gallon equals exactly 3.785411784 litres. A UK imperial gallon equals exactly 4.54609 litres, so it is about 20.1% larger. This means an identical car driven an identical distance will show a higher MPG figure in the UK than in the US. Always confirm which gallon unit you are working with before comparing fuel economy figures.
Why is my MPG lower than the EPA sticker estimate?
EPA ratings are measured on a chassis dynamometer following standardised speed and load cycles designed for comparability, not to replicate any individual driver. Real-world factors such as traffic, hills, cargo, climate control, tyre pressure, cold starts, and driving style routinely produce results 10-20% below the EPA combined figure. The EPA's own website notes this gap and offers an adjustment guide.
What is considered good fuel economy for a passenger car?
The US EPA labels any vehicle achieving 40 MPG combined or more as a fuel-economy leader. The average new light-duty vehicle in the United States achieves around 26-28 MPG combined. What counts as good depends on class: 22 MPG highway is competitive for a full-size pickup but well below average for a compact sedan.
How do I calculate cost per mile?
Divide the fuel price per gallon by your MPG. For example, at $3.50 per gallon and 30 MPG, the cost is $3.50 / 30 = $0.117 per mile. The calculator does this automatically when you enable the fuel cost toggle and enter a price.
Why use the fill-to-fill method instead of just one fill-up?
A single fill-up cannot guarantee the tank was at exactly the same level at the start as at the end. The fill-to-fill method brimmed the tank fully at both fill-ups, so the gallons pumped at the second fill exactly equal the fuel consumed over that interval. Using only one fill-up introduces an unknown starting level that can shift your MPG figure by several points.
How do I convert MPG to L/100km?
Use the formula: L/100km = 235.21 / MPG. The constant 235.21 comes from combining the miles-to-kilometres conversion (1 mile = 1.60934 km) and the gallons-to-litres conversion (1 US gallon = 3.78541 litres): 100 x 3.78541 / 1.60934 = 235.21.