I tank my car with Shell V power and get 350 to 420 miles...depending on how hard I drive.
If I do drive fast then I only see 350 odd...If i'm good I can get 400 ave.
Note:
I always drive with the traction off,
Tyre psi at 35-40,
I don't carry any unnecessary weight,
I put injector cleaner into me tank every few months,
I have replaced std air filter with a K&N one,
I only use shell V-power if I can help it.
Asda's petrol is not good..don't use it.
Nor use BP, tesco or any other supermarket chain's.
The std 95 octane bp fuel is 1.24 and V power,98 or 99 octane is near the same per ltr.
I only recommend Shell or esso...but you have to weigh the cost to the type of petrol you are getting.
Your style of driving plays a big part in it too.
I have been testing the same route to work with £20.00 pertol
Same type of driving...10 miles after the light goes on I record the mileage.
BP high octane 128.4 miles
BP std 113.1 miles
Tesco std 112.7 miles
Total high octane 123.8 miles
Total std 107.0 miles
Esso high octane 186.3 miles
Esso std 157.2 miles
Shell V power 212.8 miles
Shell std 191.6 miles
Route includes, M4 motor way, country lanes, A roads, B roads, speed humps and stop start traffic.