About this tool
MPG (miles per gallon) is the US standard for fuel economy. The metric world uses L/100km (lower is better - opposite intuition). The two relate by L/100 = 235.215 / mpg. Real-world city fuel economy is typically 70-80% of EPA highway figures.
How it works
Enter the distance you drove and the gallons of fuel used (typically the gallons it took to refill at the pump). The calculator returns mpg, metric equivalents, and projected annual fuel cost.
Fuel economy units explained
| Unit | Definition | Used in |
|---|---|---|
| MPG (US) | Miles per US gallon | United States |
| MPG (UK / Imperial) | Miles per UK gallon | United Kingdom |
| km/L | Kilometers per liter | India, Japan, several Asian countries |
| L/100 km | Liters per 100 km (lower is better) | Europe, Canada, Australia, NZ |
| mi/kWh or kWh/100mi | Electric efficiency | US EV reporting |
| Wh/km | Watt-hours per km (EV, lower is better) | EU EV reporting |
US vs Imperial gallon
1 US gallon = 3.785 L. 1 UK gallon = 4.546 L. The Imperial gallon is ~20% larger, so MPG numbers in the UK look ~20% better than the same car measured in the US. A 40 MPG (UK) car is roughly 33 MPG (US).
Cost per mile / km
Total cost of vehicle ownership goes well beyond fuel. Average US sedan in 2026:
| Cost component | Annual cost (15,000 mi) | Per-mile |
|---|---|---|
| Fuel (gasoline, 28 MPG, $3.60/gal) | $1,929 | $0.129 |
| Insurance | $1,800 | $0.120 |
| Maintenance + tires | $1,200 | $0.080 |
| Depreciation (straight-line, 5 yr) | $5,000 | $0.333 |
| Registration + state fees | $200 | $0.013 |
| Total | $10,129 | $0.675 / mile |
That's ~$0.67/mile or ~$10,000/year for a typical car. EVs save ~$1,500/year on fuel + maintenance but have higher up-front cost and faster early depreciation.
How driving habits affect economy
- Speed: most cars peak around 50-55 mph (80-90 km/h). Going from 60 to 75 mph drops MPG by 15-25%.
- Acceleration: aggressive launches use 30%+ more fuel than gradual ones.
- Idling: zero MPG. Over 10 seconds of idle uses more fuel than restarting.
- Tire pressure: 10% under-inflation costs ~2% MPG and shortens tire life ~25%.
- Roof load: aerodynamic drag costs ~5 MPG at highway speed for a roof box; ~10% for a roof basket loaded with cargo.
- Cold starts: short trips (under 5 minutes) get the worst MPG because the engine never warms up.
EV vs gas: when does the math work?
The break-even depends on miles driven, fuel price, and electricity rate. Rough rules:
- Driving over 12,000 mi/year: EV wins on running cost.
- Home charging at $0.13/kWh: EV per-mile cost ~$0.04 vs $0.13 for gas.
- Purchase premium of $5,000-$10,000 for the EV: typically recouped in 4-7 years of average driving.
- Federal tax credits (US $7,500), state credits, and free public charging shift the math substantially.
The formula explained
This calculator uses the following formula:
MPG = miles / gallons; L/100km = 235.215 / mpg
The reason this formula works is rooted in the underlying physics, finance, or biology of the problem. Behind every calculator is a published, peer-reviewed equation or a widely accepted convention. We do not invent formulas; we apply standard ones from textbooks, government tables, professional bodies, and academic literature.
If you are curious about the math, the simplest way to verify is to plug in two known numbers and compare against a known result. The calculator should match published examples to within rounding precision.
Frequently asked questions
Why is my MPG worse than EPA?
EPA tests are best-case. Cold weather, short trips, heavy loads, and aggressive driving each cost 5-15%.
How to improve mpg?
Inflate tires correctly, drive 55-65 mph instead of 75-80, smooth acceleration, remove roof racks, light foot on gas. Each saves 3-10%.
EV equivalent?
EVs use kWh per 100 mi. A Tesla Model 3 at 25 kWh/100 mi at $0.15/kWh = $3.75 per 100 mi vs $14 for a 25 mpg gas car.
Hybrid vs gas vs EV by cost?
Hybrid saves 30-40% on fuel. EV saves 60-75% on fuel but has higher purchase price. Break-even depends on use.
Why is real-world MPG lower than the EPA rating?
EPA tests use specific city/highway/combined cycles in a controlled lab. Real driving includes more idling, more aggressive acceleration, A/C use, and higher speeds. Real-world MPG is typically 10-20% below the EPA combined rating.
How accurate is the trip computer in my car?
Modern trip computers are typically within ±5% of true MPG. Some cars consistently overestimate by ~3% (Toyota tends to be optimistic; Honda is closer). For accuracy, compare miles driven to gallons pumped over 3-4 fill-ups.
Does premium gas improve MPG?
Only if your engine is tuned for premium (most performance cars, some turbocharged engines). Otherwise no - the octane rating describes detonation resistance, not energy content. Regular gas in a car designed for regular is identical to premium in MPG.
Are EVs really cheaper to maintain?
Yes. No oil changes, no spark plugs, no timing belt, no exhaust, regenerative braking reduces brake pad wear. Average EV maintenance cost is ~40% lower than ICE. Tires wear slightly faster due to weight.
How long does an EV battery last?
Modern EV batteries warrant 8 years / 100,000 miles with most retaining 80%+ capacity. Real-world data through 2025 shows degradation around 2% per year - so 80% capacity at year 10.
How accurate is the MPG Calculator?
It applies the standard formula. Accuracy is limited only by your input precision. For decisions with material consequences (taxes, medical, legal, structural), use the result as a starting point and verify with a qualified professional in the relevant field.
Is the MPG Calculator free to use?
Yes. 100% free, no signup, no payment, no API key. The site is funded by display ads around the tool but not inside the calculation flow.
Are my inputs saved anywhere?
No. All inputs stay in your browser tab. Closing the tab discards them. The site uses Google Analytics for traffic measurement (anonymized) but the analytics never see what you type into the form.
Can I use the MPG Calculator on my phone?
Yes. The tool is responsive and tested on iOS Safari, Android Chrome, and major desktop browsers. Touch targets meet Apple's 44pt and Google's 48dp minimum.
Does the MPG Calculator work offline?
Yes. Once the page has loaded, it works without internet. The calculation runs in JavaScript on your device.
How do I report a bug or suggest improvement to the MPG Calculator?
Email hi@3tej.com with the URL of this page and a description of what you saw vs expected. We typically respond within 72 hours.
Can I share results from the MPG Calculator?
Take a screenshot or copy the output. The page doesn't generate shareable URLs for specific calculations - inputs stay in your browser only.
Why are the results different from another mpg tool?
Most likely: different formula assumptions, different default values, different rounding rules, or different applicable rates. Check the methodology if both tools document it. Both can be valid for different scenarios.