of course you then have to convert the money spent into litres as petrol prices vary....
So the best way is to fill the tank to FULL. (this is the easiest place to get back to consistently).
Then drive the car till most of the tank is used. eg when light comes on.
Then fill up to full. You then know how many litres you have used. work out the gallons (litres/5*1.1) then simply gallons / miles covered is mpg.
eg.
50 litres used, divided by 5 = 10, multiply 10 by 1.1 = 11 gallons.
330 miles covered. 330/11 = 30mpg
(so for those in leons, you need to be getting around 360 miles to a tank to get 30mpg as a tank is around 55 litres)
If you view the live mpg you will always get low mpg when accelerating, especially if in low gears or accelerating aggessively.
Constant speed is good for mpg, as is anticipation, - easy off throttle earlier rather than using brakes give you much better mpg. (slowing down off throttle and in gear uses no fuel).