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speedometer inaccuracy

john sheehn

LawnSite Member
Columbus, OH
Hi guys, (and gals?) I have a 77 K20, the third 3/4 ton chevy I've had. I don't recall any of them ever having accurate speedometers. WHY? I currently have a 7 tooth speedo gear, I'm not sure of the axle gear ratios, although I believe them to be the original axles; I have slightly larger than stock tires, (9.50x32), and my speedo reads 15-20 mph faster than actual speed when travelling the highway. (65mph)....do they make different speedo gears, and if so, do I want a 6 tooth? 5 tooth? Someone told me there was a company out there who can make an adaptor to fit between the transfer case and the speedo cable. Does anyone know how to get in touch with this company? any help would be greatly appreciated. THANK YOU . ;-)


LawnSite Member
Clawson, Mi
I have never found any company that made an adapter. I would check with the dealer and see if they can check the RPO codes in the glove box and come up with the right gears.<br>There is a company called SPECMO in Royal OAK Michigan. they repair speedos and gauges. if you still need help. I will get there number for you.<p>


LawnSite Senior Member
VA zone 7
hello john call 1-888-41-speed if this # is still good ask for john hall about 2 years ago i changed rear end gears in my ford f 100 went from 273s to 410s searched web and found this guy he builds the adapter box that mounts on end of cable that goes in transmision i was 20mph off at 50mph got it fixed perfectly with the addapter