found this when mooching about, think its a nice way of quickly working things out....

http://www.torquecars.com/tools/uk-mpg-calculator.php

i put £35 worth of V-power in as i was low and feeling flush haha £1.06p per litre , 33 litres and i got 260 miles from it, which i didnt think was too bad? works out as 34.4mpg... which im happy with from my midly tuned williams! haha

whos got the best and with what mods?

Mods -

De-cat & mongoose stainless
Gunner chip
8mm HT leads
Hillpower heat shielded induction kit
all new dizzy cap and rotor arm, plugs etc...

that was with a combined motorway and urban driving