How Many Miles per Gallon does my Car Get: Worked Example, Tables & Improvement Tips
Working out how many miles per gallon does my car get is one of the most useful things you can do with a car, because it turns vague feelings about fuel use into a number you can track and improve. Below is the formula, a complete worked example, the brim-to-brim measuring method, and the conversion tables between MPG, litres per 100 km and km per litre.
how many miles per gallon does my car get: The MPG formula
MPG is miles driven divided by gallons used — for the example below, 280 miles and 7.9 gallons:
MPG = miles driven ÷ gallons used
That is the whole formula. If you drove 280 miles and used 7.9 gallons, your MPG is 280 ÷ 7.9 = 35.4 MPG. The formula works with any distance and any amount of fuel, as long as both are measured over the same period — a single tank, a week, or a month. The catch is the gallon: in the US it is 3.785 litres and in the UK it is 4.546 litres, so always say which gallon your figure uses.
The metric equivalent is litres per 100 km, and the conversions are — at 35.4 MPG they give 6.6 L/100km (US) and 8.0 L/100km (UK):
- L/100km = 235.21 ÷ MPG (US gallons) — at 35.4 MPG, that is 6.6 L/100km
- L/100km = 282.48 ÷ MPG (UK imperial gallons) — at 35.4 MPG, that is 8.0 L/100km
- km per litre = MPG × 0.4251 (US) or MPG × 0.3540 (UK) — at 35.4 MPG, that is about 15.0 km per litre (US)
At 35.4 MPG that is 6.6 L/100km on the US conversion, 8.0 L/100km on the UK conversion, and about 15.0 km per litre (US). The rest of this page uses the US gallon for worked examples unless it says otherwise, and the conversion table below gives both.
how many miles per gallon does my car get: Worked example: brim-to-brim measurement
The most accurate way to measure MPG is the brim-to-brim method, and the worked example below follows it exactly. The example uses 280 miles and 7.9 gallons.
- Fill the tank completely and note the odometer reading (or reset the trip meter to zero) before a 280-mile run.
- Drive normally until the tank is at least half empty — the longer the better, because a longer measurement averages out small errors. In the example below, the run is 280 miles.
- Fill the tank completely again at the same pump if you can, and note how much fuel went in — 7.9 gallons in the example.
- Note the distance covered since the first fill — 280 miles here.
In this example the car covered 280 miles and took 7.9 gallons to refill, so MPG = 280 ÷ 7.9 = 35.4 MPG. In metric terms that is 6.6 litres per 100 km (US conversion) or 8.0 litres per 100 km (UK conversion). Repeat the measurement over three or four tankfuls and average the results: a single tank can be thrown off by a pump that clicks off early, a different pump, or a different fill level, but an average over several tanks is a reliable picture of your car's real consumption.
Why brim-to-brim rather than the dashboard figure? The trip computer in most cars estimates consumption from the fuel injectors and is usually optimistic by a few percent. The brim-to-brim figure is a measurement of fuel actually bought against distance actually driven, which is the number that matters for your budget. If the two disagree by a lot, trust the brim-to-brim figure and use it in the fuel-cost formula. On the 280-mile example, the brim-to-brim figure is 35.4 MPG — use that, not the dashboard estimate.
how many miles per gallon does my car get: MPG to litres per 100 km conversion table
| MPG | L/100km (US gal) | L/100km (UK gal) | km per litre (US) |
|---|---|---|---|
| 15 MPG | 15.7 | 18.8 | 6.4 |
| 20 MPG | 11.8 | 14.1 | 8.5 |
| 25 MPG | 9.4 | 11.3 | 10.6 |
| 30 MPG | 7.8 | 9.4 | 12.8 |
| 35 MPG | 6.7 | 8.1 | 14.9 |
| 40 MPG | 5.9 | 7.1 | 17.0 |
| 45 MPG | 5.2 | 6.3 | 19.1 |
| 50 MPG | 4.7 | 5.6 | 21.3 |
Read the table by finding your MPG in the left column. A car doing 25 MPG (US) uses 9.4 litres per 100 km; the same car quoted in UK gallons would be 30 MPG and 9.4 L/100km on the UK column — the litres per 100 km are the same physical consumption, only the MPG number changes with the gallon. Litres per 100 km is the clearer unit for comparing cars across countries, because it does not depend on which gallon you mean. Your 35.4 MPG from the example above is 6.6 L/100km on the US column.
What changes your MPG for how many miles per gallon does my car get
Your MPG is not a fixed property of the car — it is a result of how, where and when you drive. Short trips in cold weather are the worst case: the engine uses extra fuel while it warms up, so a car that only ever does five-mile trips will show much worse MPG than the same car on a long motorway run. Speed matters too, because aerodynamic drag rises with the square of speed — cruising at 75 mph uses noticeably more fuel than cruising at 60 mph. Under-inflated tyres add rolling resistance and cost you MPG on every mile, which is why the door-jamb placard pressure is worth checking monthly on cold tyres. A roof box or roof bars add drag, extra weight adds consumption, and air conditioning adds load on the engine, particularly in stop-start traffic. None of these is dramatic on its own, but together they explain why two drivers in identical cars can report MPG figures several points apart. On the 280-mile example, that is the difference between 35.4 MPG and a figure several points worse.
Driving style is the factor you control most directly. Smooth acceleration, anticipating traffic so you brake less, and keeping a steady speed all raise MPG; harsh acceleration, late braking and unnecessary idling all lower it. The difference between a gentle driver and an aggressive one in the same car can be 10–20%, which is larger than the difference between many cars. If you are trying to improve your MPG, measure it brim-to-brim for a few tanks first, then change one thing at a time and measure again — that way you know what actually helped. For a typical family car at 32 MPG, the same logic applies: the 280-mile example's 35.4 MPG is the figure to beat, so measure first, then change one thing at a time.
Common mistakes for how many miles per gallon does my car get
The most common mistake is comparing US and UK MPG figures as if they were the same unit — they are not, and the 20% difference is enough to make a thirsty car look economical. The second is trusting a single tank: one fill can be off by a gallon or more if the pump clicks off early, so always average over several tanks. The third is measuring over too short a distance, where a small error in the fuel figure becomes a large error in the MPG. The fourth is forgetting that the official MPG figure for a car is measured on a standard test cycle, not in real driving — real-world MPG is usually worse, especially on short trips. And the fifth is using the dashboard figure as if it were a measurement: it is an estimate, and it is usually optimistic. Measure brim-to-brim, average over several tanks, and state which gallon you are using, and your MPG figure will be one you can rely on. On the 280-mile example, a single-tank error of one gallon would move the result from 35.4 MPG to 31.5 MPG — which is why averaging over several tanks matters.
How to improve your MPG for how many miles per gallon does my car get
Start with the free things: check tyre pressures monthly against the placard figure, remove roof boxes and roof bars when they are not needed, and take unnecessary weight out of the boot. Then look at how you drive: accelerate gently, anticipate traffic, keep a steady speed, and switch the engine off rather than idling for more than a minute. On a long trip, cruising at 60 mph rather than 75 mph saves a useful amount of fuel, and planning the route to avoid stop-start traffic helps too. Servicing matters as well — a clogged air filter, worn spark plugs or dragging brakes all cost MPG, and a car that has not been serviced is unlikely to be doing its best. Finally, measure: without a brim-to-brim baseline you cannot tell whether any change actually helped, so keep a simple log of miles and gallons for a month and compare. For a typical family car, the worked example's 35.4 MPG is the baseline to beat.
how many miles per gallon does my car get: Related pages
- Fuel cost calculator — turn your MPG into a trip cost for how many miles per gallon does my car get
- MPG calculator — work out how many miles per gallon does my car get with your own numbers
- Fuel cost per mile calculator — what each mile costs at 35.4 MPG
- MPG — all MPG guides and calculators, including how many miles per gallon does my car get
Frequently asked questions
How do I calculate MPG?
Divide the miles driven by the gallons used: MPG = miles ÷ gallons. For example, 280 miles on 7.9 gallons is 35.4 MPG. Measure brim-to-brim — fill up, note the mileage, drive, fill up again — and average over three or four tankfuls for a reliable figure. Say which gallon you are using: a US gallon is 3.785 litres and a UK gallon is 4.546 litres.
What is a good MPG?
There is no single good figure, because it depends on the size and type of car — a small hatchback and a large SUV have very different consumption. The useful comparison is against your own car's past figures and against similar cars measured the same way. What matters most is whether your MPG is stable: a sudden drop can point to under-inflated tyres, a dragging brake, or a service item that needs attention. For reference, the worked example on this page uses 35.4 MPG for a typical family car.
Why is my real MPG worse than the official figure?
Official MPG figures are measured on a standard laboratory test cycle, not in real driving. Short trips, cold weather, air conditioning, higher speeds and stop-start traffic all make real-world MPG worse than the test figure, and the gap is larger for cars that mostly do short urban trips. Your brim-to-brim figure is the real one for your driving — use that for budgeting. The example on this page measured 35.4 MPG brim-to-brim — that is the figure to use, not the brochure number.
How do I convert MPG to litres per 100 km?
Divide 235.21 by the MPG figure for US gallons, or 282.48 by the MPG figure for UK imperial gallons. At 35.4 MPG that is 6.6 L/100km (US) or 8.0 L/100km (UK). The table above gives the conversion for the most common MPG values.
How can I improve my MPG?
Check tyre pressures monthly against the placard, remove roof boxes and extra weight, drive smoothly with gentle acceleration and early anticipation of traffic, and avoid idling. On motorways, cruising at a steady 60 mph rather than 75 mph saves fuel. Measure brim-to-brim before and after any change so you can see what actually worked. Measure a typical family car over a few tanks first: the example here is 35.4 MPG, and any change should be judged against your own baseline.