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How do you identify proper speedometer vs rear end ratio


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My 1973 2002 apparently has had an equipment swap of either the speedometer or the rear end ratio. I came to realize the situation when highway cruising at a reading of 65mph and cars would honk at me!  After plugging in my Garmin, I discovered that I was actually traveling 9-10mph slower than what the speedometer read. I removed my speedometer and it had these numbers imprinted: W=128     020/146/2     5.73.  I haven't checked but I believe my rear end ration is  3.64:1. with 175/70 tires on 13" rims. Do I have the proper speedometer installed? If not, what designator speedmeter number  do I need? Thanks,

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On 4/27/2017 at 6:07 PM, Whaledriver said:

My 1973 2002 apparently has had an equipment swap of either the speedometer or the rear end ratio. I came to realize the situation when highway cruising at a reading of 65mph and cars would honk at me!  After plugging in my Garmin, I discovered that I was actually traveling 9-10mph slower than what the speedometer read. I removed my speedometer and it had these numbers imprinted: W=128     020/146/2     5.73.  I haven't checked but I believe my rear end ration is  3.64:1. with 175/70 tires on 13" rims. Do I have the proper speedometer installed? If not, what designator speedmeter number  do I need? Thanks,

 

I believe the 1.28 is correct for a 3.64 diff and the stock 165/80/13 tires -- slightly larger than your 175/70's, so there's a bit of error introduced there.  Any possibility that your car's original 3.64 was swapped for a 3.90 (1976 49-state '02's), 3.91 (e21's), or 4.11 (1600-2's generally) diff?  But also bear in mind....speedometers weren't always correct from the factory and speedometers fail!

 

My '76 was one of a large bunch of 1976-model speedometers that was marked 1.39 on the back but actually contained 1.28 gearing -- from the factory.  Oops.  I've had a 9-10% speedometer error from new -- of course I didn't believe the factory manufacturing error affected my car until I bought a GPS!

 

(The "5.73" indicates your speedometer was completed May 1973.  Does that mesh with your car's manufacturing date, i.e., May 1973 or later?)

 

Regards,

 

Steve

 

Edited by Conserv

1976 2002 Polaris, 2742541 (original owner)

1973 2002tii Inka, 2762757 (not-the-original owner)

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