What it computes
Three things at once:
- Fuel economy: how far you went per unit of fuel
- Trip cost: how much you spent at the pump
- Cost per mile/km: useful for budgeting and tax deductions
The calculator shows your fuel economy in all four common units simultaneously: US MPG, UK MPG, km/L, and L/100km. So whether you’re comparing a US sticker rating to a European spec sheet, the numbers translate directly.
US gallon vs UK gallon
A common pitfall: 30 MPG in the US is NOT the same as 30 MPG in the UK. The UK gallon (Imperial gallon) is 4.546 liters; the US gallon is 3.785 liters. UK MPG numbers are always 20% higher than US MPG numbers for the exact same vehicle.
This is why importers sometimes seem to inflate MPG claims, they’re quoting UK MPG to US buyers (or vice versa). The calculator distinguishes the two so you can compare apples to apples.
L/100km is the European standard
US drivers think in MPG (“higher is better”). Europe and most of the world use L/100km (“lower is better”). The math is just inverted, but the cognitive model is different, fuel economy thinking is “how much do I need to go this distance?” rather than “how far does this fuel get me?”
A modern compact car is about 8 L/100km (≈ 30 US MPG / 36 UK MPG / 12.5 km/L). Hybrids hit 4-5 L/100km. Pickup trucks and SUVs cluster at 10-13 L/100km.
How to use this for budgeting
Multiply your annual mileage by the cost-per-mile. So at 12,000 miles/year and $0.15/mi, fuel costs you ~$1,800/year. That’s the “fuel” line in a household budget.
For business mileage deductions (US 2024 rate: $0.67/mi for self-employed), this calculator shows your actual cost per mile, which is usually less than the IRS standard mileage rate. The IRS rate includes depreciation; just-fuel cost is much lower.
Frequently asked questions
Why does my dashboard MPG differ from my calculation? Dashboard estimates are computed from instantaneous fuel flow and don’t always match the average over a tank. For accurate measurement, fill up, drive normally, fill up again, divide miles by gallons used.
Should I use highway or city MPG? Sticker MPG ratings are EPA test cycle averages, neither pure highway nor pure city. For real-world planning, take the EPA combined number minus 5-10% for honest expectation.
How can I improve gas mileage? Drive smoother (fast acceleration kills economy), keep tires properly inflated (5% gain), avoid roof racks when not needed (10-15% loss at highway speed), and don’t carry excess weight (1% loss per 100 lbs). The biggest gain is usually slowing down, going 75 instead of 65 mph drops economy 10-15%.
Does fuel grade matter? For most cars: no. Use whatever the manual recommends. Premium fuel in a regular-fuel car gives no benefit and costs more. Premium-required cars need premium, using regular degrades performance and may damage the engine over time.