Read your car through a fill-up log
The quickest way to understand your own fuel economy is to write it down. Below is an example log for one car over eight fill-ups. Every figure in the table is worked out from the miles and gallons, and the notes show why the numbers swing. A roof box, a cold week and city driving with the AC on all land below the average.
Example log
Starting odometer 024180
30.2 MPG overall
| Fill | Odometer | Miles | Gallons | MPG | Against average | What was going on |
|---|---|---|---|---|---|---|
| 1 | 024492 | 312 | 10.4 | 30.0 | Mixed commute | |
| 2 | 024790 | 298 | 10.6 | 28.1 | Cold week, lots of short trips | |
| 3 | 025195 | 405 | 11.2 | 36.2 | Highway trip at about 65 mph | |
| 4 | 025566 | 371 | 12.9 | 28.8 | Highway at about 75 mph, roof box on | |
| 5 | 025871 | 305 | 10.1 | 30.2 | Mixed commute, tires set to placard | |
| 6 | 026133 | 262 | 10.3 | 25.4 | City errands, AC on high | |
| 7 | 026463 | 330 | 10.5 | 31.4 | Mixed commute, gentler starts | |
| 8 | 026781 | 318 | 10.2 | 31.2 | Mixed commute |
You can keep the same kind of log with a notebook or a phone note. The tracking guide shows the method step by step and has a calculator that converts between MPG and liters per 100 km.
Guides
What affects fuel economy
Speed, weight, drag, engine load and trip length, and how much each one usually matters.
HabitsDriving habits that save fuel
Smooth starts, earlier braking, steady speeds and less idling. The free gains are all here.
ChecksTire pressure and mileage
Where to find the right number, how to check it cold, and why pressure drops in winter.
UpkeepMaintenance that matters
Which service items affect mileage on a modern car, and which ones mostly don't.
MeasureTracking your mileage
Work out real MPG at the pump, convert units, and keep a log for your records.
ElectricEV range basics
Why range changes with temperature and speed, and how to read kWh per 100 miles.
The biggest mileage drains
These are typical reductions published in US government fuel economy guidance, rounded. Your own results depend on the vehicle, the roads and the weather, so treat them as a sense of scale. Pick any row to read more.
- Hard acceleration and brakingabout 10 to 40%Rapid starts and late braking turn fuel into heat at the brakes. The loss is largest in stop-and-go traffic.
- Large roof cargo box at highway speedabout 6 to 17%A blunt box on the roof adds drag, and drag grows quickly with speed. A rear-mounted carrier costs far less.
- Freezing weather, short tripsabout 15 to 24%A cold engine runs rich until it warms up, and short trips never let it get there.
- Tires below the placard pressureabout 1 to 3%Soft tires flex more and roll with more resistance. Losses add up slowly as pressure drops.
- An extra 100 pounds of cargoabout 1 to 2%Every pound has to be accelerated. Smaller cars feel the same load more than large ones.
- Motor oil heavier than specifiedabout 1 to 2%Thicker oil takes more effort to pump and shear. Use the grade your owner's manual lists.
0%Share of mileage lost, on a scale to 40%40%
Three numbers worth knowing
- MPG. Miles per gallon is distance divided by fuel. Higher is better. It's the figure on US window stickers and most dashboards.
- L/100 km. Liters per 100 kilometers is fuel divided by distance, so lower is better. To convert, divide 235.2 by the MPG figure. A car that gets 30 MPG uses about 7.8 L/100 km.
- kWh/100 mi and MPGe. Electric vehicles are rated by the energy they use. MPGe treats 33.7 kWh of electricity as equal to one gallon of gasoline. See EV range basics.
Common questions
Why is my mileage lower than the sticker?
Official ratings come from standard lab tests. Real driving adds weather, traffic, hills, higher speeds and extra weight, so most drivers see some gap. A short trip in winter can fall well below the rating. The factors guide walks through the usual causes.
What is the single biggest thing I can change?
For most people it's how they drive. Smoother acceleration, earlier braking and a slightly lower highway speed cost nothing and often make the largest difference. See driving habits.
Do you cover electric and hybrid cars?
Yes, in general terms. The EV range page explains how electric range works, and hybrids come up throughout, since they respond to many of the same habits.