The needle isn't proportional to what is actually in the tank. I always find the last quarter goes out the window a hell of a lot faster than the first quarter does so the only real way is to do back to back fill ups or use the odometer and run it till very nearly empty after you know how much fuel you stuck in and give yourself a small percentage for rounding error if you don't brim it but then it is never going to be that accurate anyway. Defo something wrong with 21mpg...they are only a 1.6 aren't they in a tiny little KA? That said if it does 70mph at 4000rpm that might be your problem on a long run

Although still don't explain why it would do 21mpg in normal driving!