Diff and Final Drive ratios

[paul.heneghan@bbc.co.uk (Paul Heneghan)]

Help, chaps and chapesses.

I have been trying to work out what the gearing ratio (GR) is between engine 
revs and wheel revs.

I have deduced that the GR must be about 0.315, i.e. 1 engine rev gives 
0.315 wheel revs.  This would cause 5800rpm to be geared down to 1827 rpm at 
the wheels.  If the circumference of a wheel is 1.85m, then the speed of the 
car will be 56m/s.  This is about 200km/h which is about 126mph.  That seems 
reasonable

HOWEVER, if you multiply a typical final drive figure of 4.9 by a typical 
5th gear ratio of 0.537, you get 2.6313.  That's way out.

What am I doing wrong?

Paul
paul.heneghan@bbc.co.uk
1984 Audi 80 quattro
1989 Audi 100 Avant