How to manually measure fuel consumption, on the road, in automobiles.
1
 Make sure that your vehicle is regularly serviced and is not mechanically defective in any way.
2
 Fill your fuel tank to the point at which the automatic shut off on the pump nozzle is activated. Filling the tank to near overflowing will saturate the vapour recovery system, possibly damaging it.
3
 Reset trip odometer to zero.
4
 Drive for a while.
5
 Fill the fuel tank again to the same level, noting how much fuel (in litres) you put in the tank.
6
 Make a note of the reading of the trip odometer on your fuel receipt.
7

Divide the trip odometer reading by the number of litres put into the tank.
For example: 329 miles divided by 47 litres = 7 miles per litre.

8
 There are 4.54 litres in an Imperial gallon so multiply the miles per litre by 4.54.
For example: 7 x 4.54 = 31.78 miles per gallon
9
 Record your findings in a simple spreadsheet file. Please click here for an example.
N.B.

External environmental factors can significantly effect fuel consumption figures. Therefore base fuel consumption data needs to be collated over as long a period of time as possible. Wind, air temperature and air density all impact on fuel consumption in automobiles.

In a laboratory environment, with the use of a rolling road, it is possible to measure the flow rate and pulse width of the injectors to determine fuel consumption more accurately. The manufacturer should be contacted to determine the on/off response time of an injector.

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