Go for a drive on a full tank return and use the miles to calculate the mpg then just fill up before you run out.
I've work out that i can get from Brighton to Ninove on a tank, as i spent a fortune on a new sender and gauge then a week cleaning the tank and lining it with slosh and the thing still doesn't work!
I get 3/4 down to 1/4 for the top half of the tank then empty for the rest.