jay67fast
Gearhead Posts: 146
From: Boca Raton, Florida,U.S.A.
Registered: Nov 2004
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posted 03-03-2006 01:37 PM
 
My 67, 289 C4 fastback originally had ( I think) a 2.79 open 8"rear end. Somewhere along the line a 3.25 Lim. Slip 9" rear end was installed. I decided to take advantage of the “quick change” feature and have replaced the 3.25 lim. Slip with a 2.47 open pumpkin. The car now cruises at a very reasonable RPM (relative to MPH) and doesn’t scream at 75 MPH. I am keeping the 3.25 for possible use in the future.
When the original rear was replaced the speedo gear doesn’t appear to have been changed, as the speedo was inaccurate. Now that I have a third different rear end ratio in the car, the speedo is way, way off!! I’ve been able to chart different speeds by using a calculator, and converting RPM’s to MPH, but that means having a chart in the car that shows what MPH I’m going at a given RPM. That’s a pain!! I’m at a loss as to what speedo gear should be used. I think I need a 16 tooth driven gear to bring everything back to accurate, but I’m not sure. Does anyone out there know how to calculate what driven gear I should have on the speedo cable? Is there a calculator or a formula to figure this out? By the way I am using 205P70/14 tires.
Thanks in advance for any help you can give me.
Jay67fast
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