This is one of my pet hates - so many miles to a tank...

There are so many things that can be different - how much was left in the tank when you filled it up? It will never be the same as how much was in there when someone else filled it up - you can't compare!

Here's what you do:

fill the car up until the pump clicks off.
Reset the clock
Drive round until you've used as much fuel as you want to
Fill up again, until it clicks off. Write down how much fuel you've just put in.

work out how many miles per gallon you've done - this you can compare.

miles done divided by no of litres used, then multiply by 4.54

I get about 20mpg round town, and 30-35 on a motorway run.