Fuel Efficiency Calculator
Estimate your vehicle's fuel efficiency in MPG and calculate CO₂ emissions from a trip. Ideal for road-trip planning, comparing vehicles, or tracking driving costs.
About this calculator
Fuel efficiency is measured in miles per gallon (MPG), calculated as MPG = miles ÷ gallons. To estimate CO₂ emissions, the calculator multiplies gallons consumed by a fuel-specific emission factor: gasoline produces approximately 19.64 lbs of CO₂ per gallon, while diesel produces approximately 22.38 lbs per gallon. So CO₂ (lbs) = gallons × emission factor. These factors come from the EPA's measured carbon content per gallon of each fuel type. Diesel's higher emission factor reflects its greater energy density per gallon. Knowing both MPG and emissions helps drivers make informed decisions about vehicle choice, trip planning, and their environmental footprint.
How to use
Suppose you drove 350 miles and used 14 gallons of gasoline. First, MPG = 350 ÷ 14 = 25 MPG. Next, CO₂ emissions = 14 × 19.64 = 274.96 lbs of CO₂. If you had used diesel instead: CO₂ = 14 × 22.38 = 313.32 lbs. Enter your miles driven, gallons used, and select your fuel type. The calculator instantly returns your MPG and total emissions, so you can compare fuels or trips side by side.
Frequently asked questions
How do I calculate my car's fuel efficiency in miles per gallon?
Fuel efficiency (MPG) is calculated by dividing the total miles driven by the number of gallons of fuel consumed: MPG = miles ÷ gallons. For example, driving 300 miles on 12 gallons gives 25 MPG. This is the standard measure used in the US to compare vehicle efficiency. The higher your MPG, the less fuel — and money — you spend per mile.
Why does diesel produce more CO₂ per gallon than gasoline?
Diesel fuel contains more carbon atoms per gallon than gasoline, giving it a higher energy density and a higher CO₂ emission factor — about 22.38 lbs per gallon versus 19.64 lbs for gasoline. However, diesel engines are typically more efficient, so they may travel farther on each gallon, partially offsetting the higher per-gallon emissions. The net environmental impact depends on the vehicle's actual MPG. For long-distance hauling, diesel often comes out ahead on a per-mile CO₂ basis.
When should I use a fuel efficiency calculator instead of my car's onboard display?
Your car's onboard display estimates fuel use in real time but can be off by 2–5% compared to actual pump fill-ups. Using a fuel efficiency calculator with real fill-up data — exact miles driven and exact gallons purchased — gives you a more accurate picture over time. It's especially useful when comparing vehicles, tracking fuel costs on a road trip, or verifying whether a recent tune-up improved your mileage. It also lets you calculate CO₂ emissions, which most dashboards don't show.