Speedo not right.......not even close!

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Nov 9, 2005 | 10:31 AM

vette64772

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Speedo not right.......not even close!

Thanks for any help in advance! My car is '77 with auto tranny. When I am cruising at 40 the speedo reads 80.......30 reads 60....etc. I am assuming I need to change the drive gear in the tranny. How do I determine which gear to replace it with? Don't want to buy one of each until I find the right one if I don't have to!

Nov 9, 2005 | 10:44 AM

TedH

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From: Tampa Bay FL

Have a change in rear gear ratio or rear tire height contributed to this? Did it ever work properly? New tranny? Am wondering how it got so out of synch to begin with.

I think I have a 'purple' speedo gear sitting in my parts bin that I removed from a '71 Nova after switching from something line 3.08's to 3.55's. I can't get to it until Nov 18 (in KC) but if you find that the purple gear is correct, I can send it for the price of shipping/handling.

Nov 9, 2005 | 11:15 AM

vette64772

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Thanks Ted. I got the car about 10 years ago, and it has always been that way. I am guessing there was a tranny change at some point along the way. Any idea how to guestimate which gear is the right one?

Nov 9, 2005 | 11:36 AM

TedH

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From: Tampa Bay FL

Quote:

Originally Posted by vette64772

I thought that a link to the www.keislerauto.com website would help but I couldn't find it. If you know your rear tire size (ex, P225/60R15), rpm at mph (ex. 2500rpm at 50mph), you can plug the resulting tire height (P255/60R15 tire height is 27 inches according to the keisler speed analyzer) the rpm and speed into a formula to get gear ratio. Others may have the easy formula.

Nov 9, 2005 | 12:42 PM

mooneyd

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From: Flanders NJ

You need a driven gear that's twice as many teeth as the one thats in there now. But I don't think it will fit, so you would need to change both gears. What ever ratio you have now, you need to double it but keep the gear sizes with in the limits needed to fit into trans.
Why did you wait so long? You're racking up mileage at double what you are actually driving.

Nov 9, 2005 | 09:13 PM

lowbuck72

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Joined: Jun 2005

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From: Chino Hills CA

There's an inexpensive easy fix out there. There's a guy on ebay selling speedometer ratio adapters for $9.99. You just screw the adapter into your tranny, and screw your cable into the adapter. Any it doesn't matter what ratio you buy. The adapter comes apart and there's four little gears inside you can change around to create the desired ratio. The hardest part is calculating what you need. Just pick a measured mile using highway mile markers, time yourself at an indicated 60 mph, and calculate your error. Then count teeth and put the adapter together with a ratio that will compensate and correct. Worked like a charm on both my Chevy's.

Nov 9, 2005 | 09:38 PM

David Ey

Burning Brakes

Joined: Sep 2004

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From: Ft.Worth Texas

Or call your friendly chevy dealer and tell hin what you have and he will tell you the proper gearset. Sometimes the pan must be removed so you can change the inside gear.

Nov 9, 2005 | 11:07 PM

JohnFromVentura

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From: Ventura Calif

Happen to know what your rear end ratio is? I think a 2:1 speedo error is beyond the reach of a speedo gear change. Possible your speedometer head itself has problems and needs to be remagnetized?
Regards, John