Gas Mileage Calculator – Fuel Efficiency
Calculate your fuel efficiency in MPG or L/100km and estimate fuel costs.
How to Calculate Fuel Economy (MPG / L/100km)
To measure your actual MPG: fill your tank completely, reset the trip odometer to zero. Drive normally. Fill up again completely. Divide miles driven by gallons used: MPG = miles / gallons.
Example: filled up, drove 312 miles, needed 10.4 gallons to fill up again: 312 / 10.4 = 30 MPG. For L/100km: fill up, drive, fill up again. L/100km = (liters used × 100) / km driven. A vehicle using 6L to drive 100 km gets 6 L/100km.
Factors That Affect Fuel Economy
Speed: Fuel economy peaks at 45–55 mph for most vehicles. Above 60 mph, aerodynamic drag increases rapidly — every 10 mph above 50 reduces MPG by roughly 7–14%. Acceleration: Aggressive acceleration and hard braking waste fuel. Smooth acceleration and anticipatory braking can improve MPG by 10–30%. Tire pressure: Underinflated tires increase rolling resistance. Keeping tires at the recommended PSI can improve MPG by 0.5–3%. A/C: Air conditioning can reduce MPG by 5–25% in hot weather, more on short trips.
Annual Fuel Cost Calculator
To estimate your annual fuel cost: (Miles per year / MPG) × price per gallon = annual cost.
If you drive 15,000 miles/year at 28 MPG with gas at $3.50/gallon: (15,000/28) × $3.50 = $1,875/year. Improving to 35 MPG saves (15,000/28 − 15,000/35) × $3.50 = $135/year. Over 5 years: $675 — enough to evaluate whether a fuel-efficient upgrade pays off.
Frequently Asked Questions
What is a good MPG for a car?
EPA ratings (2024): Economy cars: 30–40 MPG combined. Midsize sedans: 28–35 MPG. SUVs: 22–30 MPG. Trucks: 18–25 MPG. Hybrids: 45–55 MPG. EVs: 100–130 MPGe. The national average for new vehicles is around 28–30 MPG.
Does ethanol fuel (E85) give worse mileage?
Yes. E85 (85% ethanol) contains about 27% less energy per gallon than gasoline. Flex-fuel vehicles running on E85 typically see 15–27% lower MPG. Unless E85 is priced significantly cheaper per gallon, the fuel cost per mile is usually higher.
How much does idling affect fuel economy?
An idling vehicle gets 0 MPG and burns 0.2–0.8 gallons per hour depending on engine size. Modern fuel-injection engines consume less fuel when restarted than during 10+ seconds of idling. Turn off your engine if waiting more than 30–60 seconds.