What Affects MPG: Explained Simply, With the Maths Shown and the Caveats Stated

What affects mpg is one of those questions that sounds simple until you try to answer it for your own car. The answer depends on the car, the conditions and how you drive, so there is no single figure that fits every driver. What this page does instead is give you the method, so you can work out your own answer and know exactly what it means.

For what affects mpg, the point is this: The sections below take the question in order: the explanation first, then the step-by-step method, then the things that can go wrong and the signs that it is time to see a professional. Every number that depends on a place or a rule states the jurisdiction it applies to, because limits change and a figure that is right in one country can be wrong in another.

What you need to know first — what affects mpg

On this page, what affects mpg is the case in point: Fuel economy is a measurement, not a feeling, and the reliable way to measure it is the brim-to-brim method: fill the tank to the first click, reset the trip meter, drive normally, then fill to the first click again and divide the miles driven by the gallons used. That gives the car's real miles per gallon over those miles, in the conditions you actually drove in. The dashboard figure is a useful guide, but it is an estimate from the car's own sensors, and it can read a little high or a little low.

The useful thing about what affects mpg is that it can be checked with tools most drivers already own: a gauge, a tread-depth measure and the handbook for the car.

How it works in practice — what affects mpg

Taking what affects mpg as the case in point: The gallon matters more than most drivers realise. A US gallon and an imperial gallon are different sizes — an imperial gallon is about 20 percent larger — so the same car shows a higher MPG figure in the UK than in the USA without using any less fuel. Litres per 100 kilometres is the same consumption expressed the other way round: a lower number means less fuel used. The conversion is litres per 100 km equals 282.48 divided by imperial MPG, or 235.21 divided by US MPG, and the gallon type must be stated every time.

Worked example — what affects mpg

For what affects mpg, the point is this: Take a driver covering 16,000 miles a year as an example figure. The method on this page works the same at any mileage, so the example is there to show the arithmetic, not to state a fact about your car. Start with the measurement or the figure you can check yourself — a gauge reading, a tread-depth reading, or the figure on the placard — and write it down before you do anything else. Then apply the method step by step, checking each result against the one before it, because an error early in the chain is the one that is hardest to spot later. If the result surprises you, check the inputs first: a pressure read on warm tyres, a tread depth measured in one spot only, or a figure taken from the fitted tyre instead of the placard will each give a wrong answer that looks plausible. That is why the worked example on this page states its assumptions openly, and why you should do the same with your own figures.

What changes the answer — what affects mpg

Taking what affects mpg as the case in point: Real-world MPG differs from official figures because official figures are measured in a laboratory test cycle, while real driving includes cold starts, short trips, traffic, hills, air conditioning and the weight of whatever is in the car. A car that returns a good figure on a steady motorway run can return a much lower figure on short town trips, and both figures are honest — they just describe different driving. The way to use MPG figures is to compare figures measured the same way, over the same kind of driving.

  • On this page, what affects mpg is the case in point: Driving type: steady motorway driving usually returns better MPG than stop-start town driving.
  • Cold starts: a cold engine uses more fuel, so short trips hurt the average more than long ones. That is the point this page works through for what affects mpg.
  • Load and roof boxes: extra weight and a roof box increase drag and fuel use. That is the point this page works through for what affects mpg.
  • On this page, what affects mpg is the case in point: Tyre pressure: under-inflated tyres increase rolling resistance and fuel use.
  • Air conditioning: running the air conditioning adds load to the engine and uses more fuel. That is the point this page works through for what affects mpg.
  • For what affects mpg, the point is this: Speed: fuel use rises sharply at high speed because aerodynamic drag grows with the square of speed.

Common mistakes — what affects mpg

  • Taking what affects mpg as the case in point: Comparing a UK imperial MPG figure with a US MPG figure as if they were the same unit.
  • Trusting a single tank of fuel instead of averaging over several tanks. That is the point this page works through for what affects mpg.
  • Taking what affects mpg as the case in point: Forgetting to reset the trip meter at the fill, which adds miles from the previous tank.
  • On this page, what affects mpg is the case in point: Comparing a laboratory test figure with a real-world brim-to-brim figure and concluding the car is faulty.

Step by step — what affects mpg

  1. For what affects mpg, the point is this: Fill the tank to the first click of the pump and reset the trip meter to zero.
  2. For what affects mpg, the point is this: Drive normally until the tank is low, noting the miles on the trip meter.
  3. On this page, what affects mpg is the case in point: Fill the tank to the first click again and note the gallons shown on the pump.
  4. Divide the miles driven by the gallons used to get the MPG for that tank. That is the point this page works through for what affects mpg.
  5. For what affects mpg, the point is this: Repeat over several tanks and average the results for a reliable figure.
  6. For what affects mpg, the point is this: Convert to litres per 100 km if you need it, using the conversion for the gallon type you measured in.

What can go wrong, and when to see a professional — what affects mpg

Most problems with what affects mpg announce themselves early if you know what to look for: uneven wear, a vibration that was not there before, a warning light, a noise that changes with speed, or a reading that does not match the placard. None of these is a reason to panic, and all of them are reasons to check the basics first — pressures with a gauge when the tyres are cold, tread depth across the whole tread, and the figures on the placard rather than the ones on the fitted tyre. If the basics are right and the symptom is still there, or if you find a bulge, a deep cut, a cracked sidewall or a tyre that is losing pressure quickly, that is the point to see a tyre professional rather than to keep driving and hoping. A professional can see the inside of a tyre, measure the alignment angles and test the sensors, and those are the checks that cannot be done from the driveway. The cost of an inspection is small beside the cost of a tyre worn out early or a problem found too late.

Related guides — what affects mpg

Frequently asked questions

What affects my car's MPG most?

For what affects mpg, the type of driving matters most: short, cold, stop-start trips use far more fuel per mile than steady motorway driving. After that, tyre pressures, extra weight, roof boxes, air conditioning and speed all move the figure. Check the pressures first, because it is the cheapest fix.

How do I calculate my car's MPG by hand?

For what affects mpg, fill to the first click, reset the trip meter, drive normally, then fill to the first click again. Divide the miles driven by the gallons the pump shows. That is the brim-to-brim method, and averaging over several tanks gives a more reliable figure than a single tank. State whether you used US or imperial gallons.

Why is my fuel economy worse than the official figure?

For what affects mpg, official figures are measured in a laboratory test cycle, while real driving includes cold starts, short trips, traffic, hills and air conditioning. Short town trips in particular return much lower MPG than a steady motorway run. The official figure is a comparison tool between cars, not a promise for your driving.

Why do UK MPG figures look higher than US MPG figures?

For what affects mpg, because the gallons are different sizes. An imperial gallon is about 20 percent larger than a US gallon, so the same car travelling the same distance on the same amount of fuel shows a higher MPG number in the UK. Litres per 100 kilometres avoids the confusion because it does not use gallons at all.

How do I convert MPG to litres per 100 km?

For what affects mpg, divide 282.48 by the imperial MPG figure, or 235.21 by the US MPG figure. For example, 40 imperial MPG is about 7.1 litres per 100 km. Always state which gallon you started from, because the two conversions give different answers.