19-10-2018, 01:26 PM
The tables in Appendix 2 of Austin Seven Specials by Bill Williams are very useful.
They give the mph per rpm in top gear for all variants of axle ratio and tyre size.
You don't say what axle you have nor what tyres.
'Standard' values are:
4.9:1 with 3.50x19 tyres gives 15.83 mph per 100rpm (1923-1931)
5.25:1 with 4.00x17 tyres gives 14.29 mph per 1000rpm (1932-1936)
5.125:1 with 4.00x17 tyres gives 14.63 mph per 1000rpm (1937-1939)
5.625:1 with 3.50x19 tyres gives 13.76 mph per 100rpm (Nippy)
Changing wheel/tyre size may not make much difference:
5.25:1 with 3.50x19 tyres gives 14.75 mph per 1000rpm
As a general rule Sevens are overgeared and benefit from lower axle ratios.
Of all the Sevens I've owned the Nippy gave by far the best cruising and was happy at 65mph.
My 1929 saloon goes well at 50mph. It has an almost standard engine but flywheel is lightened and engine balanced.
However specials may well behave differently.
Highly tuned cars are easily capable of over 70mph so may need higher gearing.
They give the mph per rpm in top gear for all variants of axle ratio and tyre size.
You don't say what axle you have nor what tyres.
'Standard' values are:
4.9:1 with 3.50x19 tyres gives 15.83 mph per 100rpm (1923-1931)
5.25:1 with 4.00x17 tyres gives 14.29 mph per 1000rpm (1932-1936)
5.125:1 with 4.00x17 tyres gives 14.63 mph per 1000rpm (1937-1939)
5.625:1 with 3.50x19 tyres gives 13.76 mph per 100rpm (Nippy)
Changing wheel/tyre size may not make much difference:
5.25:1 with 3.50x19 tyres gives 14.75 mph per 1000rpm
As a general rule Sevens are overgeared and benefit from lower axle ratios.
Of all the Sevens I've owned the Nippy gave by far the best cruising and was happy at 65mph.
My 1929 saloon goes well at 50mph. It has an almost standard engine but flywheel is lightened and engine balanced.
However specials may well behave differently.
Highly tuned cars are easily capable of over 70mph so may need higher gearing.
Jim