The basic formula for petrol consumption

To calculate how much petrol your car uses, divide the distance you drove by the amount of petrol you put in the tank. If you drove 300 miles and used 10 gallons, your consumption is 30 miles per gallon (MPG). The math is: distance ÷ fuel used = consumption rate.

This method works whether you measure in miles and gallons (US or imperial) or kilometres and litres — the principle stays the same. The result tells you how many miles (or kilometres) your car travels on one unit of fuel. A higher number means better fuel economy; a lower number means the car uses more petrol to cover the same distance.

Key Takeaways

  • Divide the distance driven by the amount of petrol added to get your consumption rate in miles per gallon or kilometres per litre.
  • Fill the tank completely before and after each test period to get the most accurate reading, or use the trip meter and fuel gauge together.
  • Real-world consumption varies based on driving habits, road conditions, traffic, and weather — highway driving usually shows better economy than city driving.
  • Tracking consumption over several fill-ups smooths out the effect of a single trip and reveals whether your car's efficiency is changing.
  • Compare your actual consumption to the manufacturer's figures to spot mechanical problems or driving patterns that waste fuel.

Step-by-step calculation with a full tank method

The most reliable way to measure consumption is to fill your tank completely, reset your trip meter to zero, and drive normally until you need fuel again. When you refuel, note the distance on your trip meter and the number of gallons (or litres) the pump shows you added. Divide distance by fuel: if your trip meter reads 250 miles and you added 8.5 gallons, your consumption is 250 ÷ 8.5 = 29.4 MPG.

Repeat this process over at least three fill-ups to get a realistic average. A single calculation can be skewed by a longer highway trip or a week of heavy city traffic. When you average three or more fill-ups, you see the true pattern of how your car performs under your normal driving conditions.

Make sure the pump nozzle clicks off at the same point each time — some drivers top up an extra squeeze or two, which throws off the math. Fill to the same level before each test period begins so you are measuring the same amount of fuel each time.

Using the fuel gauge and trip meter without a full tank

If you do not want to fill the tank completely, you can use your fuel gauge and trip meter together. Set your trip meter to zero when your fuel gauge reads, for example, three-quarters full. Drive until the gauge drops to one-quarter full, then note your trip meter distance. You have now used roughly half a tank.

Check your car's fuel tank capacity in the owner's manual or on the manufacturer's website. If your tank holds 14 gallons and you used half of it, you burned 7 gallons over the distance shown on your trip meter. Divide distance by 7 to get your consumption rate.

This method is less precise than a full-tank fill-up because fuel gauges are not perfectly accurate, especially in the middle ranges. The gauge may drop faster or slower than the actual fuel level changes. For a rough estimate it works, but for tracking whether your car's efficiency is improving or declining, the full-tank method is more reliable.

Why your consumption varies between trips

The same car will show different consumption rates depending on how and where you drive. Highway driving at steady speeds typically shows better economy — sometimes 5 to 10 MPG higher — than city driving with frequent stops, starts, and idling. Cold weather, hilly terrain, heavy traffic, and aggressive acceleration all increase fuel use.

Carrying extra weight in the boot, towing a trailer, or driving with underinflated tyres also reduces economy. A roof rack or roof box creates wind resistance and can lower consumption by several miles per gallon. If you notice a sudden drop in consumption — say, from 32 MPG to 26 MPG over several fill-ups — check your tyre pressure, look for mechanical problems, or review whether your driving habits have changed.

This is why tracking consumption over time matters more than a single calculation. One trip tells you almost nothing; a pattern over weeks or months tells you whether your car is running normally or whether something needs attention.

Comparing your actual consumption to the manufacturer's figure

Car manufacturers publish fuel economy figures for new vehicles — often listed as "combined" (a mix of highway and city), "highway," and "city" consumption. You can find these on the Monroney label (the window sticker) when the car is new, in the owner's manual, or on the manufacturer's website.

Your real-world consumption will almost always be lower than the manufacturer's combined figure. This is normal. The test conditions used to generate those figures do not match everyday driving — they use standardized speeds, no idling, and optimal conditions. If your consumption is 15 to 20 percent lower than the published figure, your car is performing as expected.

If your consumption is significantly worse — say, 30 percent below the manufacturer's figure — it may signal a problem. Low tyre pressure, a faulty oxygen sensor, a clogged air filter, or worn spark plugs can all reduce economy. A mechanic can run a diagnostic to check whether the engine is running efficiently.

Tracking consumption over months and years

Keep a straightforward log of each fill-up: the date, distance driven since the last fill-up, amount of petrol added, and the calculated consumption. A spreadsheet or even a notebook works. Over time, this log shows whether your car's efficiency is stable, improving, or declining.

A gradual decline in consumption over years is normal as an engine ages and components wear. A sudden drop — especially if it happens over just a few fill-ups — is worth investigating. It may mean a mechanical issue is developing, or it may straightforward reflect a change in your driving pattern or the season (winter driving typically uses more fuel than summer).

If you are considering selling the car, this log also gives you real data to share with potential buyers about the car's actual fuel economy under normal use.

Converting between different measurement systems

If your car's manual uses kilometres and litres but you want to calculate in miles and gallons, or vice versa, use these conversions: one mile equals 1.61 kilometres, and one US gallon equals 3.785 litres. To convert MPG to kilometres per litre, divide MPG by 2.352. To convert kilometres per litre to MPG, multiply by 2.352.

For example, if you calculated 8 kilometres per litre, multiply by 2.352 to get 18.8 MPG. If your car shows 28 MPG, divide by 2.352 to get 11.9 kilometres per litre. Most modern cars sold in the UK use litres and kilometres, so you may not need to convert, but the formula is useful if you are comparing your car to vehicles sold in other markets.

Frequently Asked Questions

Should I calculate consumption on highway trips or city driving?

Calculate over your normal mix of driving — the commute, errands, and occasional longer trips you actually do. This gives you realistic consumption for your situation. If you want to know how your car performs specifically on the motorway or in town, track those separately, but your everyday average is what matters for budgeting fuel costs.

Why does my consumption seem worse in winter?

Cold engines burn more fuel before they warm up, and cold air is denser, creating more drag. Heating the cabin also draws power from the engine. Winter tyres have more rolling resistance than summer tyres. All of these factors combine to reduce economy by 5 to 15 percent in cold months. This is normal and usually reverses when the weather warms.

What if the numbers jump around wildly between fill-ups?

Single fill-ups can vary a lot depending on whether that week included a long motorway trip or mostly short city journeys. Average at least three fill-ups to smooth out these swings. If the average itself is changing — three fill-ups average 30 MPG, then the next three average 26 MPG — that signals a real shift in efficiency worth investigating.

Can I use the on-board fuel economy display instead of calculating myself?

The display gives you a quick estimate, but it is not always accurate. Manufacturer displays often show slightly better consumption than the actual numbers you calculate by hand. Use the display as a rough guide, but verify with your own calculation every few fill-ups to catch any real changes in how your car is performing.

Does hypermiling (driving very slowly to save fuel) give an accurate consumption reading?

No. Hypermiling is not representative of normal driving, and the consumption figure you get does not reflect what you will see in everyday use. Calculate consumption based on how you actually drive — your regular speed, acceleration, and driving patterns. This number is far more useful for budgeting and spotting problems.