To check the mpg (and/or pence per mile) figure for the car accurately:
1. Fill the tank right to the very top.
2. Note down the mileage (or set the trip meter to zero).
3. Drive until quite a bit of fuel has been used (preferably at least half a tank).
4. Refill the tank to the very top, noting down how many litres of fuel it takes (and, if you want a 'pence per mile' figure, the cost as well).
5. Also note down how many miles the car has been driven between the two visits to the filling station.
To get an mpg figure, first divide the number of miles driven by the number of litres used to refuel the car. (That gives you 'miles per litre'). Then multiply by 4.546.
To get a 'pence per mile' figure, divide the cost of refilling the tank (in PENCE) by the number of miles driven.
A 1.2 litre car, driven sensibly with a mix of town roads and faster roads, should be achieving at least 40 mpg. (I get that out of my car's 1.8 litre engine).
Looking at it the other way round, the cost of using fuel for the same type of driving shouldn't exceed 16p per mile (at the most). That, of course assumes that you're paying supermarket prices and not filling up at an isolated country garage in the Scottish Highlands.