How to accurately calculate MPG

[Dahaka]

So i have an older ford, the MK5 Fiesta 2002 1.3 lx and it doesnt have a built in MPG reader.

Anyway to figure that out on my own thanks.

[TomsFocus]

Fill it to the brim.  Reset the trip counter. 

When its near empty fill to the brim again, take note of the litres used to refill and the miles covered.

Then either manually do miles/(litres/4.54) or use a website such as http://www.torquecars.com/tools/uk-mpg-calculator.php 

[m1tch]

I have a mk6, I have found that the MPG trip counters overread by around 10% - mine says 72mpg, its actually doing 64mpg etc, best to do a brim to brim measurement - fill up, reset the trip counter, then just make a note of the fuel used to brim the tank again.

[dave_k]

as has been mentioned above is the easy way. I usually take it to two clicks when brimming the tank too.

[Bobr]

I've just had a run from Nottingham to Scarborough to Wakefield and back to Nottingham.   The trip computer in my Fiesta Ecoboost 100 says 60.6 to the gallon.   It was brimmed before I set out and I'll be doing it again tomorrow to give me the exact figure.   First long run since I've had it.

[dave_k]

6 hours ago, Bobr said:

I've just had a run from Nottingham to Scarborough to Wakefield and back to Nottingham.   The trip computer in my Fiesta Ecoboost 100 says 60.6 to the gallon.   It was brimmed before I set out and I'll be doing it again tomorrow to give me the exact figure.   First long run since I've had it.

the car's own computer is usually pretty close on the longer runs like that, but its the shorter runs and mixed runs over the course of a tank it tends to read 10-15% higher.

[Bobr]

Although it reported 60.6 mpg, when I filled up this morning the actual figure proved to be 57.57 mpg.   Still quite pleased with that and not that far out.   The best I ever got out of my last Fiesta on a long gentle run up to Scotland was 50.7 mpg.

[JimmyDouglas]

Just an observation...try and fill-up at the same PUMP every time, if you are doing "manual-checks"...the ground will be at the same "level"...even if it's not quite "flat". I find that my "computer" tells me 39.9mpg whereas my (manual) calculations indicate 40.2mpg but I have only had the car 7 months and I have never "reset" the computer...the previous owner may have been more "heavy-footed"! I HAVE however noticed that when I bought the car the "average" displayed "39.8",...it has now crept-up to "39.9"...this may confirm my suspicions regarding the previous owner!

[ukcal]

I wonder why the trip computers are always wrong. All of us report at least 10% over.. 

Or is it just to please those who don't actually calculate the economy (in other words, they aren't really interested in it, other than knowing that more is better!)?..

[David73]

16 hours ago, ukcal said:

I wonder why the trip computers are always wrong. All of us report at least 10% over.. 

Or is it just to please those who don't actually calculate the economy (in other words, they aren't really interested in it, other than knowing that more is better!)?..

The system does not directly measure fuel flow directly but will be is a calculation that requires some assumptions. It will most likely to be based on the amount of time the injectors are open - on each cycle the injectors are open for so many milliseconds, according to load and fuel specific gravity. Total up the milliseconds and the number of turns of the drive shaft, apply a conversion and you are presented with a number on your dashboard. The conversion would just be an average value, not a calibrated one.