How to Calculate Fuel Economy: MPG Guide
Fuel economy is simple to measure but easy to misread. This guide covers the MPG and L/100km formulas, why the two are reciprocals and why that matters, the difference between US and Imperial gallons, and why upgrading a thirsty vehicle saves more fuel than upgrading an efficient one.
How to Calculate Fuel Economy
Measuring fuel economy takes two numbers and one division. Interpreting it correctly is where most people go wrong — and the mistake costs real money when choosing a vehicle.
The basic calculation
The reliable method is brim to brim: fill the tank completely, drive, then fill completely again and record what went in.
Miles per gallon:
MPG = \frac\textmiles driven\textgallons used
Litres per 100 kilometres:
L/100km = \frac\textlitres used × 100\textkilometres driven
Note these run in opposite directions. Higher MPG is better. Lower L/100km is better. One measures distance per fuel, the other fuel per distance.
Worked example
You fill up, reset the trip meter, drive 384 miles, and the next fill takes 12.4 US gallons.
MPG = (384)/(12.4) = 30.97 \text MPG
The same trip in metric: 384 miles is 618.0 km, and 12.4 US gallons is 46.94 litres.
L/100km = (46.94 × 100)/(618.0) = 7.60 \text L/100km
Both describe identical fuel use.
The gallon problem
A US gallon and an Imperial gallon are not the same, and the difference is large enough to change a buying decision.
- US gallon = 3.785 litres
- Imperial gallon = 4.546 litres — about 20% larger
So 30 MPG (US) equals 36.0 MPG (Imperial) for exactly the same car. A US car advertised at 30 MPG and a UK car advertised at 36 MPG are equally efficient.
MPG_imperial = MPG_US × 1.201
This trips up international comparisons constantly. Always confirm which gallon a figure uses.
Converting between MPG and L/100km
The conversion is not linear, because the two measures are reciprocals of each other.
US gallons:
L/100km = (235.215)/(MPG_US) \qquad MPG_US = (235.215)/(L/100km)
Imperial gallons:
L/100km = (282.481)/(MPG_imperial)
Where does 235.215 come from? One US gallon is 3.785411784 litres and one mile is 1.609344 km. Converting units gives 100 × 3.785411784 / 1.609344 = 235.215.
Notice the spacing. Going from 15 to 20 MPG saves 3.92 L/100km. Going from 35 to 40 MPG saves only 0.84 L/100km — the same 5 MPG improvement, less than a quarter of the fuel saved.
The MPG illusion
That non-linearity has a consequence most people get backwards.
Consider two households, each driving 12,000 miles a year, each with one vehicle to replace:
- Household A upgrades a truck from 15 MPG to 18 MPG
- Household B upgrades a car from 35 MPG to 50 MPG
Household B's improvement looks far more impressive: +15 MPG versus +3 MPG. Now compute actual fuel:
Household A: 12,000/15 = 800 gallons → 12,000/18 = 667 gallons. Saved: 133 gallons.
Household B: 12,000/35 = 343 gallons → 12,000/50 = 240 gallons. Saved: 103 gallons.
The unimpressive-looking 3 MPG gain saves more fuel than the dramatic 15 MPG gain.
This is the MPG illusion, documented by Larsen and Soll in Science (2008). Because MPG is distance per fuel, equal MPG increments do not represent equal fuel savings. It is why the US EPA added gallons-per-100-miles to window stickers, and why L/100km — being fuel per distance — is arguably the more useful measure.
The practical takeaway: replacing your least efficient vehicle saves more fuel than optimising your most efficient one.
Fuel cost per distance
What actually matters to a household budget:
\textCost per mile = \frac\textprice per gallonMPG
At \3.50/gallon and 30 MPG, that is \0.1167 per mile — about \$1,400 a year over 12,000 miles.
Improve to 35 MPG and it falls to \0.10 per mile, saving roughly \200 a year. Useful, but worth weighing against what the more efficient vehicle costs to buy.
What actually affects fuel economy
Speed. Aerodynamic drag rises with the square of velocity, and the power needed to overcome it with the cube. Most cars peak in efficiency around 50–60 mph; beyond that, economy falls away sharply. Driving at 80 rather than 65 mph typically costs 15–20%.
Acceleration and braking. Every hard acceleration converts fuel into kinetic energy that braking then throws away as heat. Smooth driving routinely improves economy by 15–30%.
Tyre pressure. Under-inflated tyres increase rolling resistance. The effect is modest — roughly 0.6% per 1 PSI below spec across all four tyres — but it is free to fix and also affects safety and tyre life.
Cold starts and short trips. An engine below operating temperature runs a richer mixture and its lubricants are thicker. Trips under two miles can return half the normal economy. This is why urban figures are always worse than highway.
Weight and load. Roughly 1–2% per 100 lb in a typical car. A roof box is worse than its weight suggests, because it wrecks the aerodynamics — often 10–25% at motorway speed.
Air conditioning. Typically 3–10%, worst in stop-start traffic. At motorway speed, open windows increase drag enough that A/C is usually the more efficient choice.
Fuel grade. Using premium fuel in an engine not designed for it gains nothing. Higher octane resists knock; it does not contain more energy. Use what the manufacturer specifies.
Why your figures differ from the official rating
Official ratings come from standardised laboratory cycles, not from real roads. Expect to see 10–20% worse in practice, for entirely legitimate reasons: test cycles use gentle acceleration, moderate speeds, no A/C, mild temperatures, no cargo, and a fully warmed engine.
Manufacturers are not necessarily misleading you — the figures exist so that different vehicles can be compared under identical conditions, which is genuinely useful. They just are not predictions of your commute.
Measuring accurately
Use several tanks, not one. A single fill-up is noisy: pump auto-shutoff varies, and so does the fuel remaining. Average across three or four tanks.
Fill the same way each time. Same station if possible, same pump behaviour, first auto-click. Topping off past the click introduces error and can damage the vapour recovery system.
Record the odometer, not the trip computer. Onboard fuel economy displays are estimates and commonly read 3–8% optimistic.
Note the conditions. Winter economy is genuinely worse — denser air, winter fuel blends, longer warm-ups, more idling. A 10–20% seasonal swing is normal and does not indicate a fault.
Common mistakes
Comparing US and Imperial MPG. A 20% error, and the most frequent one in international comparisons.
Assuming MPG improvements scale linearly. The MPG illusion above.
Trusting the trip computer. Verify against a hand calculation at least once.
Measuring over one tank. Too noisy to be meaningful.
Comparing a summer figure to a winter one and concluding something is wrong with the car.
Frequently asked questions
Why is my highway economy better than city? Constant speed avoids the acceleration-braking cycle that wastes energy, and the engine stays at operating temperature. City driving also involves idling, which returns zero miles per gallon.
Does premium fuel improve economy? Only if your engine requires it. In an engine designed for regular, higher octane provides no benefit — you are paying more for fuel with the same energy content.
How much does a roof rack cost me? An empty roof rack typically costs 2–8% at motorway speed; a loaded box 10–25%. Remove it when not in use — this is one of the largest easy savings available.
Do fuel-saving devices work? Magnets, vortex generators and "fuel line ionisers" have no demonstrated effect in controlled testing. The US EPA has tested many such devices over decades and found essentially none that deliver their claims.
Is idling worse than restarting? Modern engines use more fuel idling for over roughly 10 seconds than restarting costs. Wear concerns from restarting are largely obsolete, which is why stop-start systems are now standard.
How do I compare an EV to a petrol car? EVs are rated in MPGe, which converts kWh to a gasoline-energy equivalent (33.7 kWh = 1 gallon). It is useful for energy comparison but not for cost — electricity and petrol are priced very differently, so compare cost per mile directly instead.
Summary
Divide distance by fuel for MPG, or fuel by distance for L/100km. Confirm which gallon you are using, convert with the 235.215 constant rather than assuming linearity, and average over several tanks.
Most importantly, remember that MPG gains are not linear. The largest fuel savings come from improving your least efficient vehicle, not your most efficient one.
Track your own figures with our Fuel Economy Calculator, which handles both unit systems and both gallon definitions.