SuperDevTools
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Fuel consumption calculator: mpg, L/100 km and fuel log

Units

Fill-to-fill method: fill up until the nozzle clicks off, drive, then fill up again. Enter the amount you put in and the distance driven between the two fill-ups.

How it is worked out

Your entries and your fuel log stay saved on this device.

Using less fuel: a few pointers
  • Check tyre pressures when cold, at the value given for the vehicle.
  • On fast roads, driving a little slower usually cuts consumption.
  • Look ahead: ease off early rather than brake late, and avoid hard acceleration.
  • Take out unneeded weight, and remove a roof box or bars when not in use.

How much each habit saves depends on the vehicle, the route and the weather.

Find out what your car really uses

This fuel consumption calculator works from the fuel you actually put in the tank. It gives US and UK mpg, litres per 100 km and km per litre, costs a trip, keeps a fuel log and compares the yearly fuel bill of two vehicles. It does not supply any price: you type the one at the pump. Your log stays on your device.

How to use it

  1. "Fill to fill": enter the "Fuel added at the fill-up" and the "Distance since the previous fill-up".
  2. "Convert": fill in one box (L/100 km, km/L, US mpg or UK mpg) and the others follow. "Electric vehicle (optional)" converts miles per kWh and kWh per 100 miles.
  3. "Trip cost": distance, consumption and "Price per gallon".
  4. "Fuel log": at each fill-up, the date, "Odometer" reading, amount, price, and "Full tank" ticked or not.
  5. "Compare two vehicles": a yearly distance, then each vehicle's consumption and fuel or kWh price.

The "Units" buttons switch between US mpg (miles and gallons), UK mpg (miles, fuel in litres), L/100 km and km/L. "Copy link to this calculation" reopens the tab with your figures, never with the log.

Measuring real mpg: the fill-to-fill method

Fill the tank until the nozzle first clicks off, with no topping up. Drive as usual, then fill up the same way. The fuel you put in is what the car burned since the last fill. mpg = miles ÷ gallons.

Example: 12.5 gallons after 400 miles gives 400 ÷ 12.5 = 32 US mpg, or 7.35 L/100 km. In metric, 45 litres after 650 km is 6.92 L/100 km, which is 33.98 US mpg and 40.80 UK mpg.

The fuel log uses the same method. The first full tank is the starting point, and a partial fill-up is added to the next full one. The average is total fuel over total distance across every full-to-full period, which is fairer than averaging the periods. The chart shows each period against a dashed average line; "Export as CSV" saves a spreadsheet file that "Import a CSV" can read back.

Why US and UK mpg differ

A US gallon holds 3.785411784 litres; the imperial gallon holds 4.54609 litres, about 20% more. The same car therefore shows a higher UK mpg: 30 US mpg equals 36.03 UK mpg and 7.84 L/100 km. The conversions are US mpg = 235.2146 ÷ L/100 km and UK mpg = 282.4809 ÷ L/100 km. With mpg higher is better; with L/100 km lower is better.

Official figures versus real-world mpg

Official figures come from a standard test cycle. They help compare models, but everyday driving usually returns somewhat lower mpg: short trips with a cold engine, highway speeds, air conditioning, load, hills and cold weather all count. Your own fill-to-fill numbers reflect your driving.

Trip cost and comparing two cars

A 300-mile trip at 30 US mpg with gas at $3.20 a gallon needs 10 gallons and costs $32.00. Over 12,000 miles a year at that price, a car doing 25 mpg costs $1,536.00 in fuel and one doing 40 mpg costs $960.00: $576.00 a year apart, or $48.00 a month.

Using less fuel: careful pointers

  • Check tire pressures when cold, at the value given for the car.
  • Drive a little slower on the highway.
  • Look ahead and ease off early instead of braking late.
  • Take out unneeded weight and an unused roof box.

Savings vary with the car and the route; the fuel log shows the effect on your own fill-ups.

Frequently asked questions

Why does the trip computer show a different figure?

It estimates consumption and can be a few percent out. Measuring at the pump over several tanks is the reference.

What if I forgot to log a fill-up?

That period will look too good. Log the next fill-up as a full tank and the figures are right again from there.

Is my fuel log sent anywhere?

No, it stays in this browser on this device. To keep a copy, use "Export as CSV".

Does it work for electric cars?

Yes for conversion and comparison: tick "Electric" and enter the consumption and the price per kWh.