Gas Mileage Calculator

The Formula

Fuel economy measures how efficiently a vehicle uses fuel. MPG (miles per gallon) is the US standard. L/100km (liters per 100 kilometers) is used internationally. Lower L/100km means better efficiency.

Variable Definitions

How to Use This Calculator

1. 1

   Choose between odometer readings or direct trip distance entry.

2. 2

   Enter your odometer readings (or trip distance) and fuel added.

3. 3

   Select distance and fuel units (mi/gal or km/L).

4. 4

   Optionally enter fuel price for cost analysis.

5. 5

   Review your fuel economy, costs, and efficiency rating.

Fuel economy is calculated by dividing the distance traveled by the fuel consumed.

Understanding the Concept

Fuel economy is calculated by dividing the distance traveled by the fuel consumed. For accurate results, fill your tank completely, reset your trip odometer, drive normally, then fill up again at the same pump. Use the exact gallons pumped and miles driven. To convert between MPG and L/100km: L/100km = 235.215 ÷ MPG. The imperial gallon (UK/Canada) is 1.201 times larger than the US gallon, so MPG(Imp) = MPG(US) × 1.201. Practical example: you drive 320 miles between fill-ups and pump 12.5 gallons. Your fuel economy is 320 / 12.5 = 25.6 MPG. In L/100km: 235.215 / 25.6 = 9.2 L/100km. In Imperial MPG: 25.6 × 1.201 = 30.7 MPG Imp. At $3.50/gallon, your cost per mile is (12.5 × $3.50) / 320 = $0.137/mile, your annual fuel cost at 12,000 miles/year would be $1,644. Edge cases: for accurate MPG tracking, always use the same pump and fill to the same automatic shutoff point — different pumps may shut off at different levels. Temperature affects fuel density: gasoline pumped on a hot day contains less energy per gallon because it expands. A 20°F temperature difference can change actual fuel volume by about 1%. For hybrid vehicles, MPG calculations can be misleading because the electric motor assists at low speeds, making short trips appear very efficient while highway MPG may be lower. For plug-in hybrids, MPGe (miles per gallon equivalent) is used: 1 gallon of gasoline = 33.7 kWh of electricity. If a PHEV travels 30 miles on electric (using 10 kWh) and 30 miles on gas (using 1 gallon), the effective MPGe = 60 / (10/33.7 + 1) = 60 / 1.297 = 46.3 MPGe. For EVs, efficiency is measured in miles per kWh (typically 3-4 mi/kWh for modern EVs) or kWh/100 miles.

Frequently Asked Questions

Sources & References