Stopped by A&A last week and grabbed a new speedo gear to replace the 28 that was in my 904 lock up... which was about 30mph off. By all calculations, I needed a 32 and somehow, I'm still showing 15mph off. Dash reads 75 and GPS shows 60. Not looking for perfection, but damn!
Running a 3.27 gear and 26.5 tall (245/60R15) tire.
Spoke with Nick and I think we're gonna bump up 5 teeth. Anybody else run into this?
Assuming that "30 mph off" means from 60 mph, and this is indicating 90 at a true 60, then;
From true 60 to indicated 90
is a difference of 90/60=plus 50%.. Therefore, your speedO driven gear will need the same percentage difference.
Since the Speed-O is showing more speed than is true, it is spinning too fast and so, needs more teeth to slow it down.
In this case plus 50% equates to 28 x 1.5 = 42 teeth.
As for the second gear, the 32tooth;
from a true 60 to 75 is plus 25%, and therefore you need a gear of 32 x 1.25= 40 teeth.
Since both calculations point to 41,+/- 1 tooth, I would imagine that you need a 41 tooth gear.
HOWEVER
my 1969 service manual says that for 3.23s with a 26.5 tall tire, needs a 32 tooth gear.
therefore, I would look elsewhere for the problem. and
since both calculations lead to the same result, I think it's a safe bet that the cable is fine........
Leaving your speed-O head as the problem.
