Look at my posting re. OD today. Therin I pretty well tell what you
ask about. However to specifically answer you....if you used the non
OD 3.89 gears and installed OD, your engaged final drive would be
about 20% below the 3.89 or approx. 3.11:1
This is grossly too low for that little engine to pull with any kind
of 4th gear performance. Fuel consumption would be HIGHER than a non
OD car at say 65mph because to hold 65mph would require the throttle
being positioned quite far open, in order to generate enough HP to
hold 65mph .
With the non OD gears at 65mph the carbs are only perhaps 1/4 open
and manifold vacuum is about 13".
With the rig you are thinking about, we would end up with carbs about
3/4 open and vacuum perhaps down to 5".
High vac = economy. Slowing down the rpms does not result in better
economy in this case because we slowed it down way too far.
Dick T.
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