Hi Daniel,
I hope you had a good sleep, for there is more math ahead. :-)
I looked over your original math:
> > 3.9 diff x wheel rpm = 3.9 x 911 = 3555.45
> > 3.7 diff x wheel rpm = 3.7 x 911 = 3370.70
Unfortunately, you made a mistake in the first line.
3.9 x 911 = 3552.9, not 3555.45 !
If you redo the following math with this change, I believe you will find
that with either rearend, after the shift from 3rd to 4th, you will end up
with 4421.5 rpm on the tach (I wish they were that accurate).
What do you think?
Ulix
> >
> > 3.9 diff 3555.45 divided into 60,000 = 16.875mph per 1000rpm direct
gear
> (4th)
> > 3.7 diff 3370.70 divided into 60,000 = 17.800mph per 1000rpm direct
gear
> (4th)
> >
> With a 3.9 diff at 6,000rpm in third gear you are travelling at 12.3455mph x
> 6 = 74.073mph. When you shift to 4th in theory (the speed will bleed off a
> bit in practice) you are still at 74.073mph but the revs are now 4,389rpm.
> The rev drop is 1,611rpm.
>
> With a 3.7 diff at 6,000rpm in third gear you are travelling at 13.1171mph x
> 6 = (faster than was the case with the 3.9 diff but it is the rpm that
> matters here) at 78.70mph. When you shift to 4th in theory (blah, blah, etc)
> you are still at 78.70 but the revs are now 4,421. The rev drop is 1,579
> which is of course the wrong answer because I want it to be less but at least
> it does matter what the diff ratio is: (I think we need Ron for the Math)
> > 3.9 diff x wheel rpm = 3.9 x 911 = 3555.45
> > 3.7 diff x wheel rpm = 3.7 x 911 = 3370.70
> >
> > 3.9 diff 3555.45 divided into 60,000 = 16.875mph per 1000rpm direct gear
> (4th)
> > 3.7 diff 3370.70 divided into 60,000 = 17.800mph per 1000rpm direct gear
> (4th)
> >
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