I guess my $.02 may have some value on this thread.
I am racing a Healey 100-4 with four speed plus Overdrive. I have used both
the 22% and 28% reduction, with preference to the 22%. I am running a 4.1
rear, and without OD, I reach my redline of 5000RPM at 94MPH. Even if I
switched to the popular 3.9 rear, my top speed would be only 99MPH at the
redline. A 3.5 rear would get me 110MPH, but these gears are hard to find
and quite expensive when found. Overreving is not an option, as I have
already broke a crankshaft, probably due occasional revs up to ~5500 or more.
With the 22% OD, 3rd OD gives a nice step between 3rd direct and 4th
direct...very handy in certain situations, such as coming out of the bowl at
NHIS, or turn 3 at Lime Rock. The OD is the weak component in the drive
train however, and I do a teardown approximately once a season, rotating my
spares.
I would expect a non-overdrive transmission with a 3.5 rear would be the most
durable combination for the Healey for most tracks, but I really like the
extra flexibility the OD gives.
Jim
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