Billy M,
As has been said here (and elsewhere) there are so many variants of C172 that you should consult the actual manual dedicated to the particular airframe that you are planning to fly. There are a number of generic books available, however, giving approximations for speeds and mass but none of these should be relied upon for actually operating any given aircraft. This applies to all mass-produced aircraft, large or small.
You could try AFE or Transair on-line in the UK for the generic books.
Cheers,
TOO