Gas Mileage Calculator

Evaluate your vehicle’s trip gas mileage and compare real-world performance against manufacturer and EPA window sticker ratings. Note that US gallons

Automotive

Trip Distance Driven

miles

Distance traveled on this specific trip.

Fuel Consumed

gallons

Total volume of fuel consumed over the trip.

Gallon Standard

Select US or UK imperial gallon standard.

Manufacturer / EPA Rated MPG

MPG

Window sticker or rated MPG to evaluate actual vs expected performance.

Formula & Methodology

Trip MPG = Trip Distance (miles) ÷ Fuel Consumed (gallons)

Gas mileage represents the distance traveled divided by the volume of fuel consumed during that specific journey. The percentage difference compares actual mileage against the factory rating via ((Actual MPG - Rated MPG) ÷ Rated MPG) × 100. Expected fuel consumption at the rated baseline is calculated as Distance ÷ Rated MPG.

Trip Gas Mileage

28 MPG (US)

Overview

Evaluate your vehicle’s trip gas mileage and compare real-world performance against manufacturer and EPA window sticker ratings. Note that US gallons (3.785 litres) and UK imperial gallons (4.546 litres) differ by 20%, requiring explicit unit identification. Real-world mileage varies depending on aerodynamics, tire pressure, terrain, and driving style.

Worked Examples

350-Mile Highway Trip (12.5 US Gallons, 26.0 Rated MPG)

Trip distance 350 miles, 12.5 US gallons used, manufacturer rating 26.0 MPG.

Calculate actual gas mileage: 350 miles ÷ 12.5 US gal = 28.0 MPG (US).
Calculate mileage variance: 28.0 - 26.0 = +2.0 MPG.
Calculate percentage variance: (+2.0 ÷ 26.0) × 100 = +7.7% above rated.
Calculate fuel saved: Expected fuel (350 ÷ 26.0 = 13.46 gal) - Actual (12.5 gal) = 0.96 gallons saved.
Convert to metric: 235.215 ÷ 28.0 = 8.4 L/100 km.

Final Answer: 28.0 MPG (US) | +7.7% above rating (0.96 gal saved)

200-Mile Commute Trip (10.0 US Gallons, 25.0 Rated MPG)

Trip distance 200 miles, 10.0 US gallons used, manufacturer rating 25.0 MPG.

Calculate actual gas mileage: 200 miles ÷ 10.0 US gal = 20.0 MPG (US).
Calculate variance: 20.0 - 25.0 = -5.0 MPG (-20.0% below rating).
Calculate extra fuel used: Actual (10.0 gal) - Expected (200 ÷ 25 = 8.0 gal) = 2.0 extra gallons.
Convert to metric: 235.215 ÷ 20.0 = 11.8 L/100 km.

Final Answer: 20.0 MPG (US) | -20.0% below rating (2.0 extra gal used)

400-Mile Road Trip (16.0 US Gallons, 24.0 Rated MPG)

Trip distance 400 miles, 16.0 US gallons used, manufacturer rating 24.0 MPG.

Calculate actual gas mileage: 400 miles ÷ 16.0 US gal = 25.0 MPG (US).
Calculate variance: 25.0 - 24.0 = +1.0 MPG (+4.2% above rating).
Convert to UK MPG: 25.0 × 1.20095 = 30.0 MPG (UK).

Final Answer: 25.0 MPG (US) | +4.2% above rating | 30.0 MPG (UK)

How to Use This Calculator

Reset your vehicle’s trip odometer at the start of a trip or note your distance.

Record the exact fuel volume pumped at the end of the journey.

Enter your trip distance in miles and fuel used in gallons.

Select whether you are using US gallons or UK imperial gallons.

Enter your vehicle’s EPA or manufacturer rated MPG to see performance variance.

Reference Table

Rated MPG	Actual 20 MPG	Actual 25 MPG	Actual 30 MPG	Actual 35 MPG
22.0 MPG	-9.1% (Below)	+13.6% (Above)	+36.4% (Above)	+59.1% (Above)
26.0 MPG	-23.1% (Below)	-3.8% (Below)	+15.4% (Above)	+34.6% (Above)
30.0 MPG	-33.3% (Below)	-16.7% (Below)	0.0% (Matched)	+16.7% (Above)
34.0 MPG	-41.2% (Below)	-26.5% (Below)	-11.8% (Below)	+2.9% (Above)

Frequently Asked Questions

Why does my gas mileage change significantly between summer and winter?

Winter fuel economy drops by 10% to 20% due to several thermodynamic factors: colder air increases aerodynamic drag, engines take longer to reach optimal operating temperature, winter-blend gasoline contains less energy per gallon, and cold engine oil increases mechanical friction.

How do roof racks and extra cargo affect gas mileage?

External roof boxes, bike racks, and cargo carriers increase aerodynamic drag dramatically, reducing highway fuel economy by 10% to 25%. Carrying 100 extra pounds of cargo inside the cabin reduces fuel economy by approximately 1%.

What is the difference between US and UK gallons in gas mileage calculations?

A US gallon equals 3.78541 litres, while an imperial UK gallon equals 4.54609 litres. If you calculate gas mileage using UK gallons, the result is 20.1% higher than if calculated using US gallons for the same physical vehicle.

Does driving above 65 mph reduce gas mileage?

Yes. Aerodynamic drag increases with the square of vehicle speed. According to U.S. Department of Energy testing, each 5 mph you drive over 50 mph equates to paying approximately $0.25 to $0.30 more per gallon of fuel.

How accurate are vehicle dashboard trip computers?

Dashboard displays estimate gas mileage indirectly based on fuel injection timing and distance pulses. While convenient, they frequently report 2% to 6% higher mileage than manual calculations performed at the gas pump.