Gas Mileage Calculator

About Gas Mileage Calculator

How to Calculate MPG

The formula is straightforward:

MPG = Miles Driven / Gallons Used

The most accurate way to measure your real-world fuel economy:

1. Fill your tank completely
2. Reset your trip odometer (or note the reading)
3. Drive normally until you need fuel again
4. Fill up again and note the gallons/litres added
5. Divide the miles driven by the fuel added

Example: You drove 312 miles and put in 10.4 gallons. MPG = 312 / 10.4 = 30.0 MPG.

For a more reliable number, do this over 3-4 fill-ups and average the results. Single-tank measurements can vary by 10-15% depending on driving conditions.

MPG vs L/100km vs km/L

Different countries use different fuel economy measures:

Measure	Used In	Better Economy Is	Conversion
MPG (US)	United States	Higher number	Baseline
MPG (Imperial)	United Kingdom	Higher number	1 imp. MPG = 1.201 US MPG
L/100km	Europe, Canada, Australia	Lower number	235.215 / US MPG
km/L	Japan, India, parts of Asia	Higher number	US MPG x 0.4251

Important: UK and US gallons are different sizes. A US gallon is 3.785 litres; a UK (Imperial) gallon is 4.546 litres. So a car rated at 30 US MPG would be rated at 36 Imperial MPG. This catches a lot of people out when comparing US and UK fuel economy figures.

Quick conversion: L/100km = 235.215 / US MPG. So 30 US MPG = 235.215 / 30 = 7.84 L/100km.

Average Fuel Economy by Vehicle Type

Vehicle Type	Typical MPG (US)	Typical L/100km
Small car (Corolla, Civic)	30-38	6.2-7.8
Midsize sedan (Camry, Accord)	28-34	6.9-8.4
Small SUV/Crossover	25-32	7.4-9.4
Large SUV (Tahoe, Explorer)	18-24	9.8-13.1
Pickup truck	15-22	10.7-15.7
Hybrid (Prius, Ioniq)	45-58	4.1-5.2
Plug-in hybrid (EV mode)	80-130 MPGe	1.8-2.9

The US EPA Automotive Trends Report recorded an all-time-high real-world average of 27.2 MPG for model-year 2024 new vehicles, up 0.1 MPG on 2023 and helped along by a rising share of electric and plug-in hybrid models. Strip out BEVs and PHEVs and the gasoline-only average sits about 1.7 MPG lower. The figure has roughly doubled since the 1970s thanks to fuel injection, aerodynamics, lighter materials, and hybrid technology.

Trip Cost Calculation

To estimate fuel cost for a trip:

Fuel Cost = (Distance / MPG) x Price per Gallon

Example: A 500-mile road trip in a car that gets 28 MPG, with fuel at $3.50/gallon:

Fuel needed = 500 / 28 = 17.86 gallons. Cost = 17.86 x $3.50 = $62.50.

For the same trip in a 17 MPG truck: 500 / 17 = 29.41 gallons x $3.50 = $102.94. That is $40.44 more in fuel alone.

What Affects Fuel Economy

Your real-world MPG can be significantly different from the manufacturer's rating. The biggest factors:

• Speed: Most vehicles are most efficient at 45-55 mph. FuelEconomy.gov (US Department of Energy) estimates each 5 mph driven over 50 mph is roughly equivalent to paying an extra $0.20 per gallon. At 80 mph, aerodynamic drag alone consumes about 40% of the engine's output.
• Driving style: Aggressive acceleration and hard braking can reduce MPG by 15-30% in city driving. Smooth, gradual speed changes save fuel and brakes.
• Temperature: Cold weather reduces fuel economy by 15-25% for short trips. The engine runs rich when cold, the battery is less efficient, and winter fuel blends have slightly less energy per gallon.
• Tyre pressure: Under-inflated tyres increase rolling resistance. Every 1 psi drop below recommended pressure reduces MPG by about 0.2%. Check monthly.
• Air conditioning: A/C can reduce MPG by 5-25% depending on conditions. At highway speeds, A/C is usually more efficient than open windows (which create drag).
• Payload: Extra weight costs fuel. Roughly 1-2% MPG reduction per 100 pounds of added weight. Clear out the boot/trunk if you are carrying unnecessary items.
• Terrain: Hilly routes use more fuel even though you recover some energy going downhill. Mountain driving can reduce efficiency by 10-20%.

Vehicle Comparison: Annual Savings

The comparison mode shows how much you would save per year by choosing a more efficient vehicle. The formula:

Annual Fuel Cost = (Annual Miles / MPG) x Fuel Price

Example: Comparing two vehicles over 12,000 miles per year at $3.50/gallon:

	SUV (22 MPG)	Hybrid (50 MPG)
Gallons per year	545	240
Annual fuel cost	$1,909	$840
5-year fuel cost	$9,545	$4,200

The hybrid saves $1,069 per year, or $5,345 over 5 years. That often offsets a higher purchase price, especially if fuel prices rise.

How Much Does Real-World MPG Differ From EPA Window Stickers?

Expect about a 10-25% gap between the EPA combined figure on the window sticker and what you actually see at the pump. The EPA test cycles are repeatable lab procedures, not a driving reality check: they assume moderate temperatures, no roof boxes, no winter fuel blends, and a specific acceleration profile. Fueleconomy.gov publishes owner-reported "MyMPG" data alongside official ratings for exactly this reason - real drivers on the same model routinely miss the sticker by 3-6 MPG.

The gap is largest in two cases: short urban trips in cold weather (engine never fully warms), and steady highway cruising above 70 mph (aerodynamic drag rises with the square of speed). A 2011 EPA/DOE analysis cited in the Automotive Trends Report found cold-start fuel use can be 12-28% higher than the hot-engine baseline for the first five to ten miles of a trip. If your measured MPG is within 15% of the sticker, you are driving fairly close to the test cycle.

US vs UK Fuel Prices and Running Costs

Country	Typical pump price (early 2026)	Of which: fuel duty	Cost for 12,000 miles at 30 MPG
United States (regular)	$3.15 / US gallon	$0.184 federal + state (avg ~$0.34 total)	$1,260
United Kingdom (petrol)	£1.35 / litre (~£6.14 / imperial gallon)	£0.5295 / litre duty + 20% VAT on top	~£2,045 (30 imperial MPG)
Germany	€1.74 / litre (~€6.59 / US gallon)	€0.6545 / litre duty + 19% VAT	~€2,635
Canada (regular)	CA$1.52 / litre (~CA$5.75 / US gallon)	CA$0.10 federal + provincial carbon tax	~CA$2,300

UK prices come from the AA Fuel Price Report for March 2026; US prices from the EIA Weekly Retail Gasoline survey; German and Canadian averages from their national statistics offices. The point of the table is not that the US is cheap (everyone knows that) but that a 5 MPG improvement is worth roughly three times as much to a UK driver per mile as it is to a US driver, which changes the maths on buying a hybrid.

Is a Hybrid Worth the Premium?

Work out the payback period before you decide. A 2025 Consumer Reports analysis of 22 popular models found hybrid versions cost on average $1,200-$3,800 more than the equivalent petrol trim. Dividing that premium by your annual fuel saving gives the break-even year.

Worked example: A Toyota RAV4 Hybrid AWD carries about a $2,800 premium over the standard RAV4 AWD (roughly 39 MPG combined vs 29 MPG). At 13,500 miles a year and $3.15/gallon, the hybrid uses 346 gallons (£1,090) versus 466 gallons (£1,468) for the petrol - a $378 annual saving. Payback: ~7.4 years. Drive 20,000 miles a year and it drops to five.

If you are comparing running costs against a full EV, the electricity cost calculator helps estimate home charging costs so you can compare apples to apples.

Common Mistakes When Measuring MPG

• Using trip computer figures as gospel. Independent tests by Car and Driver have found dashboard MPG readouts over-reporting by 3-7% on many vehicles. Always cross-check with a manual fill-up-to-fill-up calculation at least once.
• Mixing Imperial and US gallons. A UK-sold car quoted as "56 MPG" sounds dramatic next to a US car at "47 MPG", but converted to the same US gallon both are close to 47. Always check which gallon you are comparing.
• Measuring over a single tank. Weather, traffic, and terrain can swing a single-tank figure 10-15%. Average at least three back-to-back tanks before drawing conclusions.
• Not topping off to the same level. Clicking the pump off at the first auto-stop gives a more consistent reference than trying to fill right up to the neck. Pick one method and stick with it.
• Ignoring altitude and fuel grade. Premium fuel does not improve MPG unless the engine is tuned to require it. Higher altitudes can boost efficiency slightly (thinner air, less drag) but only noticeably above 4,000 ft.

For detailed trip expense planning including fuel stops, try the fuel cost calculator. If you are financing a vehicle purchase, the auto loan calculator helps estimate monthly payments.

All calculations run in your browser. No data is stored or sent anywhere.

Sources

• US EPA - Highlights of the Automotive Trends Report (2025)
• FuelEconomy.gov (US DOE) - Driving More Efficiently
• US Department of Energy - Driving More Efficiently
• US EIA - Weekly Retail Gasoline and Diesel Prices
• The AA - UK Fuel Price Report
• GOV.UK - Road Fuel Consumption and UK Motor Vehicle Fleet
• Consumer Reports - Hybrids and EVs