These are the notes on speedo drive gears I collected from the Wedge
list. Thanks to all who posted these. Disregard the specific notes
about which gear to use with which axle, as TR7/8s run different size
tires/wheels, I think they are 13 inch, so the gears will be different
for MGs running 14" or 15" wheels and taller tires.
The math for my car went like this:
3.07:1 rearend, P185/65R15 Dunlops that make 865 turns
per mile (Dunlop website), BL 5 speed LT77 trans, Smiths 140 mph
speedo calibrated for 1000 turns/mile.
Tires make 865 turns/mile.
865x3.07=2655.55 turns/mile at the driveshaft.
To change 2655.55 turns/mile into 1000 turns/mile for the
speedo, we need speedo gears with a 2.655 ratio.
Green driven gear (21 teeth) / TR8 drive gear (8 teeth) = 2.625
ratio, less than 2% error.
I tested my car against a measured 5 mile odometer check
on I-5, and the odometer is bang on.
As I recall from a year ago when I actually had all of these gears on
the desk in front of me, the TR7/white drive gear does in fact have 7
teeth, and my ex-British MOD trans came with a 6 tooth drive gear,
orange I think.
Hope this helps...
Scott Pontius
'78 MGB V8
San Diego
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