A car advertised at 40 MPG may look economical until you ask which gallon the number uses. The US gallon and the Imperial gallon are different sizes, so the same vehicle can honestly have two different MPG figures.
One name, two gallon volumes
UK MPG = US MPG × 1.20095. US MPG = UK MPG × 0.832674. These factors apply to equivalent efficiency.
Worked example
A vehicle returning 30 US MPG converts to 30 × 1.20095 = 36.03 UK MPG. Reverse the calculation: 36.03 × 0.832674 = about 30 US MPG. The car did not become more efficient; the Imperial gallon is larger than the US gallon.
What changes in everyday driving
An Imperial gallon is 4.54609 litres, while a US gallon is 3.785411784 litres. Metric L/100 km avoids the ambiguity: 30 US MPG is about 7.84 L/100 km. When comparing a UK road test with a US listing, convert both to one common unit before judging fuel cost.
Common mistakes to avoid
Changing the label from US to UK without changing the number is the classic error. Another is converting MPG to L/100 km with a rounded factor and then comparing close results as if they were exact.
A quick way to sanity-check the result
Run the arithmetic in the opposite direction. If your answer says a vehicle used 56 litres, multiply the stated efficiency by the distance and see whether you arrive near that volume. Large discrepancies usually point to a unit mismatch, an omitted return leg or an input copied from the wrong vehicle. Save the original figures with the estimate so you can update one assumption without rebuilding the whole calculation.
It is also worth testing a second case. Use a conservative fuel economy, a slightly higher energy tariff, a heavier load or a longer route. Planning with one perfect number makes a result look precise while hiding the uncertainty that matters in everyday driving.
Use a second scenario, not a second guess
Once the first answer is on screen, change one assumption that could realistically move. For a journey that might mean adding a detour; for a tyre calculation it may mean comparing an approved alternative; for finance it may mean a shorter term. Keeping every other input unchanged tells you how much that single choice matters. Write down the result and the assumption beside it.
This is also a useful way to spot implausible inputs. If doubling a distance barely changes a fuel bill, something is wrong. If a small tyre-profile change produces an enormous diameter difference, check the rim conversion and percentage calculation. A quick sense check often catches a typo before it becomes an expensive decision.
What to record with the answer
Save the date, vehicle or route, the original measurement and the unit standard. That detail sounds fussy until you revisit an estimate months later and cannot remember whether the price was per litre, US gallon or Imperial gallon. It is especially helpful when several people are sharing a trip budget or when a workshop is comparing before-and-after figures.
Do not hide uncertainty behind extra decimal places. Showing 58.33 is useful when the inputs are known; showing 58.333333 does not make a fuel price or laboratory economy figure more precise. Round for the reader, retain precision in the calculation, and explain which assumptions are likely to move first.
One detail worth carrying forward
The safest workflow for an international comparison is to convert both vehicles to L/100 km, then apply the fuel price used in your own market. This avoids accidentally pairing a UK review number with a US gallon price and also makes the direction of “better” obvious: lower consumption wins.
Use the MPG conversion tools to translate a review figure before adding it to your fuel-cost estimate. For official specifications, safety limits and contract terms, use the responsible manufacturer, professional or lender document as the final authority.
Questions drivers often ask
Is UK MPG always higher?
For the same underlying efficiency, yes numerically, because an Imperial gallon contains more fuel.
What unit is least confusing?
L/100 km is widely used internationally and clearly states fuel consumed over a fixed distance.
Do dashboard MPG figures use the same standard everywhere?
No. Check the vehicle market, handbook or display settings before comparing them.
Updated September 10, 2026