Three measures, two directions
US and UK MPG measure distance per fuel (bigger is better) but use different gallons — the imperial gallon is 20% larger, so the same car scores about 20% higher in UK MPG. Most of the world instead uses L/100km, which measures fuel per distance (smaller is better). The conversions: MPG(UK) = MPG(US) × 1.20095, and L/100km = 235.215 ÷ MPG(US). A 30 MPG(US) car is 36 MPG(UK) and 7.8 L/100km.
Why L/100km thinking prevents a classic mistake
Because MPG is a reciprocal scale, equal MPG gains are not equal fuel savings. Upgrading from 15 to 20 MPG saves about 167 gallons per 10,000 miles; going from 40 to 50 MPG saves only 50. A 5-MPG improvement on a thirsty truck beats a 10-MPG improvement on an efficient sedan. L/100km makes this obvious — savings scale linearly — which is why fuel-per-distance is the better scale for comparing vehicles.
Turning economy into dollars
Annual cost = (miles per year ÷ MPG) × price per gallon. At 12,000 miles, 30 MPG, and $3.50/gal, that's 400 gallons and $1,400 a year — about 11.7 cents per mile. Real-world economy typically runs 10–20% below EPA window-sticker figures (cold starts, short trips, speed, and cargo all hurt), so estimating with a slightly lower MPG than the sticker gives a more honest budget.
How to use this converter
Type a figure into US MPG, UK MPG, or L/100km and the other two follow instantly. For the cost estimate, add your yearly mileage and local fuel price; the tool returns annual, monthly, and per-mile fuel cost — often the number that actually decides between two vehicles.
A worked example
A car rated 40 MPG(US) converts to 48 MPG(UK) and 5.9 L/100km (235.215 ÷ 40). Drive 15,000 miles a year at $3.75 a gallon and you'll burn 375 gallons for about $1,406 — roughly 9.4 cents a mile. A 30 MPG car over the same year burns 500 gallons and costs about $1,875, a $469 difference that repeats every year you own it. Nudge the MPG down 15% toward real-world figures for a more honest budget.