Hi James & Bill,
Since the speedometer is driven from the transmission output, it has no
way of knowing that you changed the axle ratio. The speedometer must be
recalibrated. As Bill said, an indicated 70 mph would actually be 16%
faster, or about 81 mph.
To calculate:
1- Measure the tire rolling circumference. Mine comes out to 79 inches
or 6.58 feet.
2- Calculate the overall drive ratio. For a 28% overdrive, mine comes
out at 3.54 x .78 = 2.76. For a 22% od, it would be 3.54 x .82 = 2.90.
NOTE: The drive ratio is 1 divided by 1.28 = .78. For a 22% od it would
be 1/1.22 = .82.
3- Calculate drive shaft revolutions per mile. At 60 mph. 5280 ft/ 6.58
ft (the tire circumference) = 802 revolutions per mile & at 60 mph, it
also = 802 revolutions per minute.
4- 802 revolutions per minute x 2.76 overall ratio = 2214 RPM at 60 mph.
5- Multiply 2214 rpm by 1.33 for 80 mph. (80/60 = 1.33) = 2944 rpm.
6- In direct drive at 60 mph, it would be 3.54 overall ratio, so 802 X
3.54 = 2840 rpm vs 2214 in od.
Dave Russell
BN2
James Lea wrote:
> The other factor I forgot to mention was that I have not yet had the
> odometer recalibrated. I just had it rebuilt and calibrated last year so
> I was reluctant to do it again. Does anyone know just how big a
> difference it will make? Last year I measured the tire rotation on the
> road and the cable rotation in the cockpit and sent the figure to
> Nisonger with the odometer. I think tomorrow I do it again and see how
> much of a difference I come up with. That might shed some light on the
> difference in the two rear ends and how it reads on the odometer needle.
> Thanks, JL
>
> James Lea
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