Your dashboard says 38 mpg. You have always believed it. Then one day you do the arithmetic on a full tank and get 34.
Nothing is wrong with the car. The dashboard was never measuring what you thought it was.
This piece covers the tank to tank method that gives your real fuel economy, why the number on the display tends to be optimistic, how to convert between the units that make international comparisons so confusing, how to do the same thing honestly for an electric car, and the patterns in the figure that tell you something needs attention.
Why the Dashboard Number Is an Estimate
A trip computer does not measure fuel leaving the tank. It estimates consumption from what the engine management system knows, mainly how long the injectors are open, and divides by the distance it thinks you have covered.
Both halves carry error:
- The fuel side is modelled, not measured. Independent testing over the years has repeatedly found trip computers reading optimistic, commonly by a few percent and sometimes more
- The distance side comes from wheel rotation. Tyre wear, non standard tyre sizes and normal odometer tolerance all nudge it
A few percent does not sound like much, but it compounds. It is the difference between thinking a car costs you a certain amount a year to run and it actually costing noticeably more. It also hides slow deterioration, because an optimistic number that drifts down slightly still looks fine.
The fix is to use the one measurement that cannot be flattered: the fuel you actually paid for.
The Tank to Tank Method
This is how fuel economy has been measured by careful owners for decades, and it needs nothing more than a pump receipt and the odometer.
1. **Fill the tank until the pump clicks off the first time.** Do not top up after the click. Consistency matters more than getting it brim full
2. **Note the odometer reading,** or reset a trip meter to zero
3. **Drive normally** until you need fuel again. Ideally use most of the tank, because a larger sample reduces error
4. **Fill again to the first click** and note how much fuel went in
5. **Divide the distance driven by the fuel added**
That result is your real economy for that tank.
Example: you drove 412 miles since the last fill, and the pump put in 11.8 US gallons. 412 divided by 11.8 is 34.9 mpg.
A single tank has noise in it, because pumps click off at slightly different levels and a car parked on a slope fills differently. **Average five or more tanks** and that noise mostly cancels. If you ever skip logging a fill, do not guess. Start the chain again from the next full fill, because one missing entry breaks every calculation after it.
The Units Problem
Fuel economy is expressed three common ways, and the confusion between them explains most arguments about whether a car is efficient.
| Unit | Used in | Direction |
|---|---|---|
| US mpg | United States | Higher is better |
| UK mpg | United Kingdom | Higher is better, but uses the larger imperial gallon |
| L/100km | Most of Europe, Australia and elsewhere | Lower is better |
The key facts:
- A US gallon is 3.785 litres. An imperial gallon is 4.546 litres. So **the same car shows about 20 percent more mpg in UK units** than in US units. A UK review quoting 50 mpg and a US review quoting 42 mpg can describe exactly the same engine
- To convert **US mpg to L/100km**, divide 235.2 by the mpg figure. 30 US mpg is about 7.8 L/100km
- To convert **UK mpg to L/100km**, divide 282.5 by the mpg figure. 50 UK mpg is about 5.6 L/100km
There is also a reason L/100km is arguably the more honest unit. Because mpg is an inverse measure, the same improvement looks very different depending on where you start. Going from 10 to 12 mpg saves far more fuel over a given distance than going from 40 to 50 mpg, even though the second jump looks bigger. Fuel per distance makes the real saving obvious.
Electric Cars: Measure at the Wall
The equivalent for an EV is miles per kWh, or kWh per 100 miles or 100 km. The trap is the same as with a petrol car's trip computer, just larger.
The car reports the energy it took out of the battery. It does not include the energy lost getting into the battery in the first place. Charging losses between the wall and the battery are real, and they vary with the charger type, the charging speed, and whether the car is warming or cooling its battery while it charges.
For an honest number:
- Use the **kWh your home charger or the public charger billed you for**, not the figure on the car's display
- Divide miles driven by kWh bought, the same way as the tank to tank method
- Expect the result to be noticeably lower than the car claims, especially in cold weather
This is also the number that tells you your actual cost per mile, which is the only figure that matters when comparing an EV with a combustion car.
From Economy to Cost per Mile
Economy is interesting. Cost is what you actually care about.
Fuel cost per mile = price paid for the fuel ÷ miles driven on it.
A tank that cost 52 dollars and lasted 412 miles cost about 12.6 cents per mile in fuel.
This number cuts through a lot of confusion. It automatically accounts for price changes at the pump. It lets you compare petrol, diesel, LPG and electric on the same basis. And it makes the effect of a habit visible. Driving at 80 instead of 70 on the motorway, or leaving a roof box on all summer, shows up as a figure you can put a price on.
Fuel is only part of the picture, of course. Servicing, tyres and repairs are the other half of what a car costs, and the maintenance schedule piece covers what is due when.
What Moves the Number
Some variation is completely normal. Knowing the innocent causes stops you chasing problems that do not exist.
Normal, usually seasonal:
- Cold weather. Engines take longer to reach efficient temperature, winter fuel blends carry slightly less energy, and heaters and demisters add load. EVs lose more in the cold because the battery itself is less efficient and cabin heating comes from the same battery
- Short trips. A car that never warms up fully uses noticeably more fuel per mile
- Traffic and hills. Stop start driving and a hillier commute both cost fuel
- Air conditioning, roof racks and extra weight
Worth investigating if the drop is sustained and your driving has not changed:
- Tyre pressure. The cheapest thing to check and one of the most common causes
- A dragging brake. Often accompanied by one wheel being hotter than the others after a drive
- Engine sensors and thermostat. A failing oxygen sensor or a thermostat stuck open can quietly raise consumption, sometimes before a warning light appears
- Air filter and spark plugs, particularly on older cars that are overdue for service
- Wrong oil grade after a service, which has a small but real effect
The key word is sustained. One bad tank means nothing. Three in a row, in similar conditions, is a signal.
Why a Running Log Beats Occasional Checks
Doing the tank to tank calculation once is informative. Doing it every fill turns fuel economy into an early warning system.
With a continuous record:
- A gradual drop becomes visible after a few tanks, not a year later
- You can separate a seasonal dip from a genuine fault by comparing with the same months last year
- You have documented evidence for a mechanic, rather than a vague sense that the car feels thirsty
- A buyer sees a car that was watched, which supports the value in the same way a full service history does
The reason most people stop is the friction. Writing down odometer readings and litres in a notebook or spreadsheet at a fuel pump is tedious, and one missed entry breaks the chain.
Where Autoteca Fits
Autoteca is a car maintenance and fuel log for iPhone, built to make this kind of record easy to keep.
You log each fill or charge in gallons, litres or kWh and the economy is worked out for you. Odometer readings are kept consistent across every record, so a typo that would ruin a spreadsheet calculation gets caught. Photograph the receipt and it reads the date and total on your device, as suggestions you confirm rather than values written without asking. It handles gasoline, diesel, LPG, hybrid and electric, and a charge is recorded in kWh rather than pretended to be a fill, with efficiency shown the way your car reads it.
The same log carries your services, so fuel and maintenance live in one place, and the cost view shows spending per month, per category and per mile. When it is time to sell, the full service history exports as a PDF.
What it does not do:
- It cannot measure what you do not enter. The economy is only as accurate as your fills. A skipped fill or a partial top up will distort that calculation
- It does not diagnose the car. It can show that economy has dropped over the last few tanks. Working out why is still a job for you or a mechanic
- It will not correct charger losses you cannot see. For an EV, logging the kWh the charger billed gives an honest figure. Logging the car's own display figure repeats the car's optimism
There is no account, no analytics and no ads. The log lives on your iPhone and syncs through your own iCloud. Autoteca Pro is a single one time purchase that adds full fuel history, the cost dashboard, PDF export and unlimited vehicles. There is no subscription, and iCloud backup is never locked behind it.
The Short Version
- Dashboard fuel economy is an estimate from the engine computer and commonly reads a few percent optimistic
- Real MPG: fill to the first click, drive, fill to the first click again, divide miles by gallons added
- Average five or more tanks, and restart the chain if you miss logging a fill
- UK mpg uses the larger imperial gallon, so it reads about 20 percent higher than US mpg for the same car
- US mpg to L/100km: 235.2 ÷ mpg. UK mpg to L/100km: 282.5 ÷ mpg
- For an EV, measure at the charger, not on the car's display, because charging losses are real
- Fuel cost per mile is price paid divided by miles driven, and it compares every powertrain fairly
- Cold weather and short trips lower economy normally. A sustained drop in similar conditions points at tyres, brakes, sensors or overdue service