Enter how far you drove and how much fuel you used — this calculator gives you your exact fuel economy in MPG, UK MPG, and L/100km instantly. I use this same fill-up method in the shop when a customer tells me their gas mileage feels off; real numbers from a receipt beat the dashboard estimate every time.
Fuel Economy Calculator
How To Use This Calculator Accurately
The most common mistake is using an estimated mileage number or a partial fill-up figure. Here’s the method that gives you accurate results every time:
- Step 1: Fill your tank completely until the pump clicks off.
- Step 2: Note your odometer reading (or reset your trip odometer to zero).
- Step 3: Drive normally until you need fuel again.
- Step 4: Fill the tank completely again — the pump receipt shows exactly how much fuel you used.
- Step 5: Select your units above, enter the distance and fuel amount, and hit Calculate.
Always fill to the same level — full tank to full tank. Partial fill-ups throw the math off. Save one fuel receipt per tank and track it for a month. If the number starts drifting down, that’s your early warning signal before any dashboard light comes on.
Understanding Your Results: US MPG vs UK MPG vs L/100km
The calculator shows three figures because fuel economy is measured differently around the world — and using the wrong standard gives you a misleading number:
- US MPG — miles per US gallon (3.785 litres). The standard in the United States. Higher is better.
- UK MPG — miles per Imperial gallon (4.546 litres). The standard in the UK and Ireland. A UK gallon is larger, so UK MPG is always about 20% higher than US MPG for the same trip — they are not interchangeable.
- L/100km — litres per 100 kilometres. The standard in Europe, Canada, and Australia. Unlike MPG, lower is better. A car at 8.0 L/100km is more efficient than one at 10.0 L/100km.
What Is a Good Fuel Economy for My Vehicle?
| Vehicle Type | US MPG | UK MPG | L/100km | Concern Below |
|---|---|---|---|---|
| Compact Car (Civic, Corolla) | 30–38 | 36–46 | 6.2–7.8 | 26 US MPG / 8.9 L/100km |
| Midsize Sedan (Camry, Accord) | 26–34 | 31–41 | 6.9–9.1 | 22 US MPG / 10.7 L/100km |
| Midsize SUV (RAV4, CR-V) | 24–30 | 29–36 | 7.8–9.8 | 20 US MPG / 11.8 L/100km |
| Full-Size SUV (Tahoe, Expedition) | 16–22 | 19–26 | 10.7–14.7 | 13 US MPG / 18.1 L/100km |
| Half-Ton Pickup (F-150, Silverado) | 17–24 | 20–29 | 9.8–13.8 | 14 US MPG / 16.8 L/100km |
| Hybrid (Prius, Insight) | 45–58 | 54–70 | 4.1–5.2 | 38 US MPG / 6.2 L/100km |
Why Is My Fuel Economy Lower Than It Should Be?
| Cause | Typical MPG Loss | L/100km Increase | Other Symptoms |
|---|---|---|---|
| Underinflated tires | 2–4 MPG | +0.5–1.2 | Uneven wear, soft feel |
| Dirty air filter | 2–5 MPG | +0.7–1.8 | Sluggish acceleration |
| Worn spark plugs | 3–6 MPG | +1.0–2.2 | Rough idle, misfires |
| Failing O2 sensor | 5–10 MPG | +1.8–3.8 | Check engine light, rough running |
| Stuck-open thermostat | 3–8 MPG | +1.0–3.0 | Temp gauge stays low, slow warm-up |
| Dirty fuel injectors | 3–7 MPG | +1.0–2.5 | Rough idle, hesitation |
| Dragging brake caliper | 2–6 MPG | +0.7–2.2 | Pulling, burning smell |
| Slipping transmission | 4–10 MPG | +1.5–3.8 | High RPM, delayed shifts |
| Bad mass airflow sensor | 5–12 MPG | +1.8–4.5 | Check engine light, stalling |
| Clogged catalytic converter | 4–10 MPG | +1.5–3.8 | Loss of power, rotten egg smell |
If you’re also seeing a misfiring engine alongside poor fuel economy, don’t ignore it — misfires dump raw fuel into the exhaust and can destroy a catalytic converter quickly.
Factors That Affect Fuel Economy That Aren’t a Repair Problem
Cold Weather
Your engine runs richer when cold and burns more fuel to warm up. In winter you can lose 10–20% of your normal fuel economy, especially on short trips where the engine never fully reaches operating temperature. This is normal and comes back when temperatures rise.
Highway vs. City Driving
City driving with constant stop-and-go burns significantly more fuel than steady highway cruising. If your mix of driving has shifted, that will show up in your numbers even if the car is perfectly healthy.
Roof Racks, Cargo, and Towing
An empty roof rack adds wind resistance. Extra weight and trailers require more fuel. These are real hits that aren’t worth diagnosing — just worth knowing about.
Ethanol Blends (E10, E15)
E10 contains about 3% less energy than pure gasoline. E15 costs less at the pump but lowers fuel economy measurably. Switching fuel grades recently can explain a small drop.
How To Calculate Your Fuel Cost Per Mile or Per Kilometre
- Cost per mile (US): Gas price per gallon ÷ US MPG → e.g. $3.40 ÷ 28 = $0.121/mile
- Cost per mile (UK): Fuel price per litre × 1.609 ÷ UK MPG → e.g. £1.50 × 1.609 ÷ 45 = £0.054/mile
- Cost per km (L/100km): Fuel price per litre × L/100km ÷ 100 → e.g. $1.60 × 8.5 ÷ 100 = $0.136/km
Worked Examples
Example 1: 2019 Toyota Camry (US driver)
Miles driven: 332. US gallons pumped: 11.6. 332 ÷ 11.6 = 28.6 US MPG (34.3 UK MPG / 8.2 L/100km). EPA estimate is 29 city / 41 highway. Mostly city driving — 28.6 is right on target. No concern.
Example 2: 2019 Ford Focus (UK driver)
Miles driven: 280. UK gallons pumped: 7.8. 280 ÷ 7.8 = 35.9 UK MPG (29.9 US MPG / 7.9 L/100km). WLTP estimate for the 1.0L EcoBoost is around 49 UK MPG combined — 35.9 is a bit low. Worth checking tyre pressure and spark plugs.
Example 3: 2021 VW Golf (European driver)
Kilometres driven: 520. Litres used: 42. (42 ÷ 520) × 100 = 8.1 L/100km (34.9 US MPG / 41.9 UK MPG). WLTP estimate for the Golf 1.5 TSI is around 5.8 L/100km. At 8.1 on mostly motorway driving, something is off — check the air filter, spark plugs, and O2 sensor.
Mechanic’s Tip
Track your fuel economy every single fill-up for 60 days. Don’t just check it once — the trend matters more than any single reading. A gradual decline of 2–3 MPG (or a rise of 1–1.5 L/100km) over 8–10 fill-ups almost always points to something specific: an O2 sensor beginning to fail, spark plugs wearing out, or a thermostat not fully closing. Catching it early is a $150 repair. Ignoring it until the drop is significant usually means a $600+ bill.
Frequently Asked Questions
Is the fuel economy shown on my dashboard accurate?
Close, but not always precise. The display is calculated from fuel injector data and MAF sensor readings — it can drift if either is slightly off. I’ve seen dashboard readings run 1–3 MPG optimistic. The fill-up method using your actual receipt is the most accurate way to know your real number.
Why is UK MPG higher than US MPG for the same car?
Because a UK (Imperial) gallon is larger — 4.546 litres versus 3.785 litres for a US gallon. A car getting 35 US MPG is getting approximately 42 UK MPG — neither number is wrong, they’re just different standards. Never compare a US MPG figure directly to a UK MPG figure.
Does AC hurt my fuel economy?
Yes — air conditioning can reduce fuel economy by 5–25%. At highway speeds, running AC is more efficient than open windows due to drag. In city traffic, windows down beats AC.
Can premium fuel improve my MPG?
Only if your engine requires it. If the owner’s manual says “premium required,” using regular will reduce both performance and fuel economy. If it says “regular,” you’re paying extra for no measurable benefit.
My fuel economy suddenly dropped significantly — what should I check first?
A sudden large drop almost always triggers a check engine light or is about to. Pull the codes with an OBD2 scanner first — a failing O2 sensor, MAF sensor, or coolant temp sensor will cause the ECU to run rich immediately. If there’s no code yet, check tyre pressure and feel for heat at each wheel after driving. Also see: how your fuel system affects range.
How many fill-ups should I track before trusting my average?
At least 3–5 fill-ups. One reading can vary based on ground level, ambient temperature, and pump shut-off timing. After 5 consistent readings you’ll have a reliable baseline.
As an Amazon Associate we earn from qualifying purchases.
