Free MPG Calculator
Calculate your car's real miles per gallon from distance and fuel used. Converts to km/L and L/100km with cost per mile.
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Fuel Economy
29.2 MPG
km/L
12.4
L/100km
8.1
Cost Per Mile
$0.120
Formula
MPG = Miles Driven ÷ Gallons Used | Cost per Mile = (Gallons × Price) ÷ MilesHow to Calculate Your Real Miles Per Gallon
Your car's dashboard MPG display is an estimate that can be off by 5-15%. The most accurate way to measure fuel economy is the tank-to-tank method: fill up completely, drive normally, fill up again, and divide the miles driven by the gallons it took to refill. This calculator does that math for you and also converts to metric units.
For the most reliable reading, track your MPG over at least 3-4 fill-ups. A single tank can be skewed by unusual driving conditions — a highway road trip will show higher MPG than a week of city errands.
What Affects Your Fuel Economy
Understanding what hurts your MPG helps you improve it. The biggest factors are:
- Driving behavior — rapid acceleration and hard braking waste fuel; smooth, steady driving improves MPG by 15-30%
- Speed — aerodynamic drag increases exponentially above 50 mph, making 75 mph driving 20-25% less efficient than 55 mph
- Tire pressure — underinflated tires increase rolling resistance; every 1 PSI below optimal costs about 0.2% MPG
- Vehicle maintenance — dirty air filters, old spark plugs, and misaligned wheels all reduce efficiency
- Air conditioning — AC can reduce MPG by 5-25% depending on conditions, but at highway speeds it's more efficient than open windows
MPG Units Explained: MPG vs. km/L vs. L/100km
The US uses miles per gallon (MPG), where higher is better. Most of the world uses liters per 100 kilometers (L/100km), where lower is better. Canada and some countries also use km/L. This calculator converts between all three so you can compare vehicles rated in different systems.
Quick reference: 30 MPG ≈ 12.75 km/L ≈ 7.84 L/100km. A car rated at 5 L/100km in Europe is equivalent to about 47 MPG — very efficient.
Improving Your MPG: Quick Wins
You can improve fuel economy by 10-20% without spending money. Keep your tires at the recommended PSI (check monthly). Remove roof racks and cargo carriers when not in use — they create drag that costs 2-8% MPG at highway speeds. Avoid idling for more than 60 seconds. Use cruise control on flat highways. And plan your errands to combine trips, since a cold engine uses significantly more fuel for the first few miles.
Frequently Asked Questions
How do I calculate my car's real MPG?
Fill your tank completely, reset your trip odometer, drive normally until you need fuel again, then fill up and note the gallons. Divide miles driven by gallons used. For accuracy, repeat this over 3-4 fill-ups and average the results.
Why is my actual MPG lower than the EPA rating?
EPA tests are done in controlled lab conditions. Real-world MPG is typically 10-20% lower due to aggressive driving, short trips (cold engine), city stop-and-go, AC use, roof racks, underinflated tires, and carrying extra weight.
What is a good MPG for a car?
For gas cars, 30+ MPG combined is good, 40+ is excellent. Most SUVs get 22-28 MPG. Trucks average 18-24 MPG. Hybrids typically achieve 45-60 MPG, and plug-in hybrids can exceed 100 MPGe when using electric mode for short trips.
How do I convert MPG to L/100km?
Divide 235.215 by your MPG. For example, 30 MPG = 235.215 ÷ 30 = 7.84 L/100km. Lower L/100km is better (opposite of MPG where higher is better). Most efficient gas cars achieve 5-6 L/100km.
Does driving speed affect MPG?
Yes, significantly. Most cars reach peak fuel efficiency at 35-45 mph. Above 50 mph, every 5 mph increase is like paying an extra $0.20-$0.30 per gallon. Driving 75 mph instead of 65 mph can reduce your MPG by 10-15%.