Measuring Real Fuel Economy
Manufacturer figures come from a standardised laboratory cycle. Your own figure comes from your own driving, your own routes and your own vehicle's condition, and it is usually 10–20% worse. Measuring it takes one tank.
MPG = Miles driven ÷ Gallons used
L/100 km = Litres used ÷ Kilometres driven × 100
The tank-to-tank method
- Fill the tank completely and note the odometer, or reset the trip meter.
- Drive normally — ideally a full tank, and at least 150 miles.
- Fill completely again at the same pump, letting it click off on its own both times.
- Divide the distance by the fuel added.
Filling to the same click point both times is what makes the measurement valid; the fuel added on the second fill is exactly the fuel used. Repeat over three tanks and average, since one tank can be skewed by a single long motorway run or a week of short cold trips.
Economy Unit Conversions
| US mpg | Imperial mpg | L/100 km | km/L |
|---|---|---|---|
| 15 | 18.0 | 15.68 | 6.4 |
| 20 | 24.0 | 11.76 | 8.5 |
| 25 | 30.0 | 9.41 | 10.6 |
| 30 | 36.0 | 7.84 | 12.8 |
| 35 | 42.0 | 6.72 | 14.9 |
| 40 | 48.0 | 5.88 | 17.0 |
| 50 | 60.1 | 4.70 | 21.3 |
| 60 | 72.1 | 3.92 | 25.5 |
Typical Figures by Vehicle Type
| Vehicle | US mpg (combined) | L/100 km |
|---|---|---|
| Full-size pickup truck | 15–20 | 11.8–15.7 |
| Large SUV | 17–22 | 10.7–13.8 |
| Midsize SUV | 22–28 | 8.4–10.7 |
| Midsize sedan | 28–36 | 6.5–8.4 |
| Compact car | 32–42 | 5.6–7.4 |
| Hybrid | 45–58 | 4.1–5.2 |
| Plug-in hybrid (petrol mode) | 40–55 | 4.3–5.9 |
Why Your Trip Computer Reads High
Most in-car economy displays are optimistic by 3–8%. They estimate fuel flow from injector pulse width rather than measuring it, and manufacturers tend to calibrate on the flattering side. The tank-to-tank method measures the fuel you actually paid for, which is why it is the figure to trust.
Improving Real-World Economy
- Slow down. Above 50 mph, every extra 10 mph costs roughly 7–14%. Drag rises with the square of speed.
- Smooth out the driving. Anticipating traffic instead of accelerating and braking hard is worth 10–40% in town.
- Check tyre pressures monthly. Every psi below specification costs about 0.2%.
- Remove the roof box when not in use. It can cost a quarter of your economy at motorway speed.
- Combine short trips. A cold engine can use twice the fuel for the first few miles.
- Do not idle. Restarting uses less fuel than idling for ten seconds.
- Keep up with servicing. A clogged air filter, worn spark plugs or a failing oxygen sensor each cost several percent.
A Note on Sudden Drops
If your economy falls noticeably without a change in driving, the usual causes are under-inflated tyres, a dragging brake caliper, a failing oxygen sensor, a stuck thermostat leaving the engine running cold, or simply winter fuel blends and cold starts. A drop of more than 15% that persists into warm weather is worth investigating.
Frequently Asked Questions
How many miles should I measure over?
At least 150, and ideally a full tank. Short samples are dominated by fill-up variation.
Why is my winter economy so much worse?
Cold engines run rich until warm, winter fuel blends contain slightly less energy, tyre pressures fall in cold air, and heaters and defrosters add load. A 12–22% drop in cold weather is normal.
Is US mpg the same as UK mpg?
No. The imperial gallon is 20% larger, so 30 US mpg equals 36 imperial mpg. Comparing the two directly makes British cars look more efficient than they are.
Does a heavier right foot really matter that much?
Yes. Independent testing consistently finds 10–40% differences between calm and aggressive driving of the same car on the same route. It is the single largest factor under your control.