Fuel Economy Converter
Switch between mpg, km/L, and L/100km — works with both US and UK gallons. You can also compare two vehicles side by side or figure out what a trip will cost in gas.
Pick a tab, enter your numbers, and hit Convert.
Conversion Formulas
| From | To | Formula | Example |
|---|---|---|---|
| mpg US | L/100km | 235.215 ÷ mpg | 25 mpg = 9.41 L/100km |
| L/100km | mpg US | 235.215 ÷ L/100km | 8 L/100km = 29.4 mpg |
| mpg US | km/L | mpg × 0.425 | 25 mpg = 10.63 km/L |
| km/L | mpg US | km/L × 2.352 | 10 km/L = 23.5 mpg |
| mpg US | mpg UK | mpg × 1.201 | 25 mpg US = 30 mpg UK |
| mpg UK | mpg US | mpg × 0.833 | 30 mpg UK = 25 mpg US |
| km/L | L/100km | 100 ÷ km/L | 12 km/L = 8.33 L/100km |
| L/100km | km/L | 100 ÷ L/100km | 8 L/100km = 12.5 km/L |
- Divide 235 by mpg to get L/100km (and vice versa)
- For km/L, multiply mpg by 0.425
- UK gallons hold 20% more than US gallons
- Higher mpg means lower L/100km
US vs UK Gallons
| Measure | US | UK (Imperial) | Difference |
|---|---|---|---|
| 1 Gallon | 3.785 L | 4.546 L | UK is 20% larger |
| 25 mpg | 9.41 L/100km | 7.84 L/100km | Same car, different numbers |
UK ratings look higher because their gallons are bigger. A car rated 25 mpg in the US shows 30 mpg on UK sites. Same car, same efficiency, different number. Check which gallon type before comparing.
Fuel Economy Ranges
| Vehicle Type | mpg US | L/100km | km/L | Efficiency |
|---|---|---|---|---|
| Sports Car / Large Truck | 15-20 | 12-16 | 6-8 | Poor |
| Large SUV | 18-22 | 11-13 | 8-9 | Below Average |
| Mid-Size SUV | 22-28 | 8-11 | 9-12 | Average |
| Mid-Size Sedan | 25-32 | 7-9 | 11-14 | Good |
| Compact Car | 30-38 | 6-8 | 13-16 | Very Good |
| Hybrid | 45-55 | 4-5 | 19-23 | Excellent |
| Electric (MPGe) | 100-140 | 1.7-2.4 | 42-59 | Outstanding |
How Fuel Economy Numbers Work
Fuel economy can be distance-per-fuel or fuel-per-distance. MPG and km/L measure how far you go on a gallon or liter — bigger numbers are better. L/100km flips it around and measures consumption — smaller numbers win. They're inverses.
The inverse math trips people up. Jumping from 15 to 20 mpg saves more gas than 35 to 40 mpg, even though both are 5 mpg jumps. That's why L/100km can be easier for comparing fuel costs.
Why Different Countries Use Different Units
The US kept mpg because that's what Americans were using when cars showed up. Canada switched to metric in the 1970s and went with L/100km. So did most other countries.
The UK is weird. Fuel pumps sell liters, road signs show miles, but official car ratings use mpg with Imperial gallons. Since Imperial gallons are 20% bigger than US gallons, a UK car review will show a higher mpg number than the same car gets in a US review.
US vs UK Gallons
A US gallon is 3.785 liters. A UK gallon is 4.546 liters, 20% more. So 30 mpg in the UK works out to around 25 mpg by US measurement. If you're comparing cars across countries, check which gallon the spec sheet uses.
Electric Vehicle Efficiency
EVs don't burn gas, so the EPA made up MPGe to compare them against regular cars. It's based on how much energy is in a gallon of gas. New EVs generally fall in the 100-140 MPGe range.
Most EV drivers ignore MPGe and track Wh per mile instead. Figure somewhere around 250-350 Wh per mile, worse in cold weather when the battery and cabin heater are fighting each other.
The calculator above handles MPGe conversions too.
Factors Affecting Fuel Economy
Window sticker numbers are optimistic. Gas cars do better on highways where you're not braking all the time. Hybrids are the opposite. They do better in city traffic because braking charges the battery.
Other stuff that eats into your mileage:
- Low tire pressure wastes gas
- Faster speeds burn more fuel, especially above 50 mph
- Extra weight in the car or cargo area
- Running the AC or heater
- Roof racks add drag even when they're empty
- Aggressive driving — hard acceleration and braking kills mileage
Doing the Math
The math is miles ÷ mpg = gallons, then gallons × price = cost. A 300-mile trip at 25 mpg and $3.50 gas costs about $42.
The Vehicle Comparison tab above does this math for you over a year. Plug in your current car against one you're considering to see the yearly difference. A 10 mpg improvement adds up fast if you drive a lot.