Unit Converters guide
The MPG Illusion: Why 10 to 12 MPG Beats 40 to 50 MPG
· 4 min read
Miles per gallon measures distance per unit of fuel, but what you pay for is fuel per unit of distance. Because those two are reciprocals, equal-looking mpg gains don't save equal amounts of fuel. Going from 10 to 12 mpg saves more fuel than going from 40 to 50 mpg. Fuel-per-distance units such as L/100 km or gallons per 100 miles show the savings directly, which is why they are the better way to compare upgrades.
Researchers call this the "MPG illusion". Here is a worked example with real numbers.
A worked comparison: which upgrade saves the most fuel?
Imagine a household with three vehicles, each driven 10,000 miles a year. They can afford to upgrade one:
- Option A: replace an old pickup getting 10 mpg with one getting 12 mpg.
- Option B: replace a sedan getting 20 mpg with one getting 25 mpg.
- Option C: replace a hybrid getting 40 mpg with one getting 50 mpg.
On paper, Option C looks best because it's the biggest jump at +10 mpg. Option A looks feeble at +2 mpg. Now divide 10,000 miles by each mpg figure to see how much fuel is actually burned:
| Upgrade | Fuel before | Fuel after | Saved per year |
|---|---|---|---|
| A: 10 → 12 mpg | 1,000 gal | 833 gal | 167 gal |
| B: 20 → 25 mpg | 500 gal | 400 gal | 100 gal |
| C: 40 → 50 mpg | 250 gal | 200 gal | 50 gal |
The "small" pickup upgrade saves more than three times as much fuel as the hybrid upgrade.
The same comparison in L/100 km
Convert each figure to liters per 100 km and the ranking is obvious straight away, with no division needed:
| US mpg | L/100 km | gal/100 mi |
|---|---|---|
| 10 | 23.52 | 10 |
| 12 | 19.60 | 8.33 |
| 20 | 11.76 | 5 |
| 25 | 9.41 | 4 |
| 40 | 5.88 | 2.5 |
| 50 | 4.70 | 2 |
Option A cuts consumption by 3.92 L/100 km, Option B by 2.35 and Option C by 1.18. Fuel-per-distance numbers can be subtracted directly, and the differences scale with what you spend at the pump. That's why most of the world uses them, and why US window stickers now print gallons per 100 miles next to mpg.
Percentages can mislead too
A "20% better mpg" doesn't mean 20% less fuel. Going from 10 to 12 mpg is a 20% increase in mpg but a 16.7% reduction in fuel used. The effect also runs the other way. A car that drops from 8 to 6 L/100 km uses 25% less fuel, yet its rating climbs from 29.4 to 39.2 US mpg, which is a 33% increase. When you compare claims, check which kind of percentage you're reading.
US mpg, UK mpg and why the same car has two ratings
An imperial gallon (4.546 L) is about 20% bigger than a US gallon (3.785 L). So the same car posts a higher mpg in Britain:
- 30 US mpg is 36.0 UK mpg, or 7.84 L/100 km.
- 40 UK mpg is only 33.3 US mpg.
- 5 L/100 km is 47.0 US mpg or 56.5 UK mpg.
If you're comparing a UK review with a US one, convert both to L/100 km first, because liters are the same everywhere.
How to convert fuel economy with the tool
- Open the Fuel Economy Converter. It starts on US mpg to L/100 km.
- Type a figure in Value.
- In From, choose your unit: "Miles per gallon (US)", "Miles per gallon (UK)", "Liters per 100 kilometers", "Kilometers per liter", "Miles per liter" or "US gallons per 100 miles".
- Choose the target in To, or press ⇄ to swap.
- Read the result. The table below shows the same figure in all six units at once, which makes it easy to build a comparison like the one above.
Because L/100 km and gal/100 mi are reciprocal units, the tool divides rather than multiplies (L/100 km is 235.21 ÷ US mpg). If you enter 0 with one of those units on either side, you'll get a message explaining that 0 has no finite result. A car that uses no fuel has no meaningful fuel consumption figure. Everything is calculated locally in your browser.
There is also a shortcut page, MPG to L/100 km, with those two units preselected.
When to use which unit
- Comparing cars or upgrades: use L/100 km or gal/100 mi. Differences between them translate directly into fuel and money.
- Estimating range from a full tank: use mpg or km/L. Multiply by the tank size to get distance.
- Reading foreign reviews: convert everything to L/100 km first, so US and UK gallons can't get mixed up.
Fuel savings depend on how you actually drive, so treat any yearly figure as an estimate, not a promise. To turn gallons saved into money, the Unit Price Calculator is a quick way to compare costs.
FAQ
Does a higher L/100 km mean a better car?
No, it's the opposite. L/100 km is fuel used, so lower is better. A 4.7 L/100 km car is more efficient than a 9.4 L/100 km one.
How do I convert km/L to L/100 km?
Divide 100 by the km/L figure. 15 km/L is 6.67 L/100 km, or about 35.3 US mpg.
Why not just compare mpg differences?
Because equal mpg steps save less and less fuel as mpg rises. Only fuel-per-distance differences line up with the fuel you actually buy.
Compare your own cars with the free Fuel Economy Converter.