How to Calculate MPG and Fuel Economy (and Read L/100km Right)

The window sticker on a new car is a lab number. It comes from a dynamometer in a temperature-controlled room with a test driver following a fixed throttle trace. Your commute has hills, cold mornings, a roof box, and the occasional sprint to make a light. So the honest answer to "what does my car actually get?" is never on the sticker — it's in your own fuel log. The good news is that measuring it takes nothing more than a trip odometer and a single division.

This guide covers the math, the unit conversions that trip everyone up, and the one signal worth watching: a sudden drop in mileage that usually means something needs attention.

The formula is one division

Miles per gallon is exactly what it says:

MPG = miles driven ÷ gallons used

If you covered 300 miles and the pump put 10 gallons back in to refill the tank, that's 300 ÷ 10 = 30 MPG. No constants, no fudge factors. The only thing that makes it inaccurate is bad inputs — a partial fill-up or a guessed distance.

Outside the US the same idea is usually expressed as litres per 100 kilometres, which is fuel per distance instead of distance per fuel:

L/100km = litres used ÷ kilometres driven × 100

Drive 500 km on 35 litres and you're at 35 ÷ 500 × 100 = 7 L/100km. Note the direction flips: a high MPG is good, but a low L/100km is good. They measure the same physical thing from opposite ends, which is the single most common source of confusion when people compare cars across markets.

Tank-to-tank beats the dashboard

I stopped trusting the trip computer in my own car after a long drive where it claimed 6.8 L/100km while my hand-logged number came out at 7.4. That's a 9% gap, and it's typical — dashboard averages can be optimistic by 5 to 10 percent, and worse, they reset themselves at the least convenient moments.

The fix is the full-tank method:

1. Fill the tank to the first click of the pump and zero the trip meter.
2. Drive a normal mix of roads for a week or so.
3. At the next fill-up, top off to the same first click and note the fuel added — that number is the fuel you used.
4. Divide trip distance by that fuel.

Doing this over two or three tanks averages out the noise from traffic, terrain, and pump cut-off variance. One half-tank measured on a single highway run will flatter you; the multi-tank average is the number that actually predicts your next fill-up. You can keep the running figures honest with the MPG Calculator, which shows MPG, L/100km, and km/L side by side so you never have to convert in your head.

US gallons, UK gallons, and why a number looks too good

Here's the trap that wrecks cross-border car comparisons: there are two gallons. A US gallon is 3.785 litres; a UK (imperial) gallon is 4.546 litres — about 20% bigger. Because the gallon on the bottom of the fraction is larger, the same car always shows a higher number in UK MPG.

A car rated 30 MPG (US) is roughly 36 MPG (UK). Same fuel burned, same distance, bigger measuring cup. If you see a used import advertised at "40 MPG" and you're shopping in a market that uses the other gallon, you can be off by a fifth before you even test drive it. Always confirm which gallon a figure uses, then convert to a neutral yardstick — L/100km, or better yet cost per mile — before you compare.

Converting between MPG and L/100km

Because MPG and L/100km are inverse measures, you can't just multiply by a fixed rate the way you would for metres to feet. The conversion uses a constant divided by your figure:

L/100km = 235.215 ÷ MPG(US)
L/100km = 282.481 ÷ MPG(UK)

So 30 MPG (US) is 235.215 ÷ 30 ≈ 7.84 L/100km. Going the other way, divide the same constant by the L/100km value: 7 L/100km is 235.215 ÷ 7 ≈ 33.6 MPG (US). The two constants come straight from the gallon-to-litre and mile-to-kilometre factors, so they're exact — there's nothing to round in the setup, only in your final reading.

The practical move is to never memorise either constant. Type your trip once, read every unit at the same time, and pick whichever one matches the spec sheet you're comparing against.

A mileage drop is a maintenance signal

The reason to log fuel economy isn't bragging rights — it's that a clean baseline turns your tank into a diagnostic instrument. When the number moves, the cause is usually one of these:

• Cold weather and short trips. The engine never reaches operating temperature, so winter mileage routinely sags 10 to 20 percent. This one is normal and recovers in spring.
• Under-inflated tyres. Every 0.2 bar low costs roughly 1 to 2 percent. A slow puncture you haven't noticed shows up here first.
• Added drag and weight. A roof rack, a loaded boot, or a bike carrier can knock off several percent at motorway speed.
• A clogged air filter or cheap winter fuel blend. Gradual, easy to miss without a log.

What you're really watching for is the sudden, unexplained drop. If your driving hasn't changed but a couple of tanks come in 20% worse, that's not weather — it points to a failing oxygen sensor, a sticking brake caliper dragging a wheel, or a misfire. Caught early from a fuel log, these are cheap fixes; left to escalate, they aren't. To turn that consumption figure into a running-cost estimate per trip, pair it with the Fuel Cost Calculator.

Put it into practice

Pick one unit and stick to it so your log is comparable tank to tank. Measure over at least two full fills before you trust the number. Convert to a neutral figure — L/100km or cost per mile — whenever you compare two cars, two fuel grades, or two markets. And keep an eye on the trend, not the single best fill-up: a stable baseline is what lets a 20% drop jump out at you before it becomes a repair bill.

The math is genuinely one division. The discipline is in measuring it the same honest way every time.

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Made by Toolora · Updated 2026-06-13