Thats generally the milleage you'll get with normal fuel but ive found using the 'super unleaded' fuels give you more mileage and better performance.
Being in the petrol trade myself i can also tell you that it is genuinally better for your engine.
I've tried Shell Optimax and also the Esso alternative but ive now found that the best one is the new BP ULTIMATE FUEL, for both mileage and performance.
I've recommended it to some friends and they all seem to agree. Give it a try and im sure you'll see the difference to and would appreciate if you could post back whether you think it does or not.
Shane
