MPG Calculator

Calculate your vehicle’s true fuel economy in miles per gallon (MPG) using the accurate tank-to-tank fill-up method. Note that 1 US gallon equals 3.78

Automotive

Previous Odometer Reading

miles

Odometer reading from your last full tank fill-up.

Current Odometer Reading

miles

Odometer reading today at the pump.

Fuel Added

volume

Volume of fuel required to refill the tank back to full.

Fuel Volume Unit

US gallons and UK imperial gallons differ by ~20%.

Fuel Price (per unit volume)

$/unit

Price per gallon or litre paid at the pump.

Formula & Methodology

MPG (US) = (Current Odometer - Previous Odometer) ÷ US Gallons Added

The tank-to-tank method measures actual consumption between two complete fill-ups. The trip distance is calculated by subtracting the previous odometer reading from the current odometer reading. Dividing distance by fuel added calculates miles per gallon. UK imperial MPG is derived using the exact ratio of 4.54609 to 3.785411784 litres. Metric consumption in L/100 km is calculated as (Litres ÷ Kilometres) × 100.

Fuel Economy

30 MPG (US)

Overview

Calculate your vehicle’s true fuel economy in miles per gallon (MPG) using the accurate tank-to-tank fill-up method. Note that 1 US gallon equals 3.7854 litres, whereas 1 UK imperial gallon equals 4.5461 litres — making UK MPG approximately 20% higher for identical fuel consumption. Real-world fuel economy varies with driving habits, weather, payload, and tyre pressure, and rarely matches laboratory window-sticker ratings.

Worked Examples

360-Mile Highway Tank Fill-Up (12.0 US Gallons)

Previous odometer 45,200 miles, current odometer 45,560 miles, 12.0 US gallons added at $3.50/gal.

Calculate distance driven: 45,560 - 45,200 = 360 miles.
Calculate US fuel economy: 360 miles ÷ 12.0 US gal = 30.0 MPG (US).
Convert to UK imperial economy: 30.0 × (4.54609 ÷ 3.785411784) = 36.0 MPG (UK).
Convert to metric: (100 × 3.785411784) ÷ (1.609344 × 30.0) = 7.8 L/100 km.
Compute total fill-up cost: 12.0 gal × $3.50 = $42.00 ($0.117 per mile).

Final Answer: 30.0 MPG (US) | 36.0 MPG (UK) | 7.8 L/100 km | $0.117/mile

280-Mile Commute Tank Fill-Up (10.0 US Gallons)

Previous odometer 12,000 miles, current odometer 12,280 miles, 10.0 US gallons added at $3.60/gal.

Calculate distance: 12,280 - 12,000 = 280 miles.
Calculate US MPG: 280 miles ÷ 10.0 US gal = 28.0 MPG (US).
Calculate UK MPG: 28.0 × 1.20095 = 33.6 MPG (UK).
Calculate L/100 km: 235.215 ÷ 28.0 = 8.4 L/100 km.
Calculate cost per mile: (10.0 × $3.60) ÷ 280 = $0.129 / mile.

Final Answer: 28.0 MPG (US) | 33.6 MPG (UK) | 8.4 L/100 km | $0.129/mile

450-Mile Road Trip (15.0 US Gallons)

Previous odometer 68,000 miles, current odometer 68,450 miles, 15.0 US gallons added at $3.40/gal.

Calculate distance: 68,450 - 68,000 = 450 miles.
Calculate US MPG: 450 miles ÷ 15.0 US gal = 30.0 MPG (US).
Calculate UK MPG: 30.0 × 1.20095 = 36.0 MPG (UK).
Calculate total fuel cost: 15.0 × $3.40 = $51.00 ($0.113 per mile).

Final Answer: 30.0 MPG (US) | 36.0 MPG (UK) | 7.8 L/100 km | $0.113/mile

How to Use This Calculator

Fill your vehicle’s gas tank completely and record your odometer reading.

Drive normally until your next fuel stop.

Fill your gas tank completely again and record the gallons pumped and the new odometer reading.

Enter both odometer readings, the fuel volume, and the fuel price per gallon.

View your calculated MPG (US), MPG (UK), metric L/100 km, and fuel cost per mile.

Reference Table

US MPG	UK Imperial MPG	L/100 km	km/L	Classification
15.0 MPG (US)	18.0 MPG (UK)	15.7 L/100 km	6.38 km/L	Heavy Truck / Large V8
20.0 MPG (US)	24.0 MPG (UK)	11.8 L/100 km	8.50 km/L	Midsize SUV / Pickup
25.0 MPG (US)	30.0 MPG (UK)	9.4 L/100 km	10.63 km/L	Standard Crossover / Sedan
30.0 MPG (US)	36.0 MPG (UK)	7.8 L/100 km	12.75 km/L	Efficient Compact Sedan
35.0 MPG (US)	42.0 MPG (UK)	6.7 L/100 km	14.88 km/L	Subcompact / Mild Hybrid
45.0 MPG (US)	54.0 MPG (UK)	5.2 L/100 km	19.13 km/L	Full Hybrid (Prius / Ioniq)
55.0 MPG (US)	66.1 MPG (UK)	4.3 L/100 km	23.38 km/L	Ultra-Efficient Plug-in Hybrid

Frequently Asked Questions

What is the difference between US MPG and UK MPG?

A US gallon is 3.78541 litres, while a UK imperial gallon is 4.54609 litres (about 20.1% larger). Because a UK gallon contains more fuel, a vehicle that achieves 30.0 MPG in the United States achieves 36.0 MPG in the United Kingdom for the exact same physical fuel consumption.

Why is the tank-to-tank fill-up method more accurate than trip computers?

Onboard dashboard displays estimate fuel economy based on fuel injector pulse widths and airflow sensors, which often read 2% to 5% optimistically. The tank-to-tank method divides actual physical distance by the precise liquid volume registered by certified gas pump meters.

Why does city driving deliver lower MPG than highway driving?

City driving involves frequent idling, accelerating from stops, and lower transmission gear ratios. Kinetic energy is repeatedly dissipated as brake heat rather than maintaining momentum, causing fuel economy to drop by 20% to 40% compared to steady-state highway cruising.

How do I convert US MPG to L/100 km?

To convert US MPG to L/100 km, divide 235.215 by your US MPG rating (e.g. 235.215 ÷ 30 MPG = 7.84 L/100 km). Note that while higher MPG indicates better fuel efficiency, lower L/100 km indicates better efficiency.

Why does my real MPG not match the EPA window sticker?

EPA testing follows standardized laboratory dynamometer cycles under controlled temperature conditions (75°F) with no wind, roof racks, aggressive acceleration, or air conditioning load. Real-world conditions like winter cold, ethanol blends, and speeds above 65 mph reduce actual mileage.