droknows

I'm moving on to the next issue with my 1972 vette. I'm trying to determine if my speedometer or speedometer cable are bad. At 30mph my car reads 50mph and 70mph my car reads 110mph and it progresses at a consistant pace between the 30mph to the 70 mph. I've lubed the speedo cable and the symptoms remain the same, before I spend the money on a new speedometer I was thinking that someone changed out the rear end. The car is an automatic and at 70 mph the tac is reading about 3500rpm. Is that normal/high/low? I'm hoping that all I need to do is replace the transmission driven gear?

johnt365

go here http://tri-5.chevyrides.com/rpm.php

stock tires and a 3.73 rear with a 1:1 end ratio auto puts you at 3300 rpm at 70. If you add a couple hundred rpm for the converter, you are at 3500. Does the car feel like it has 3.73? It would really launch pretty good if so.

This site also helps calculate the driven gear.

droknows

Thanks for the info John. I had a C5 before, so in comparison the 72 seems pretty slow. I really couldn't tell you if she launches good. I don't have all the vacuum hoses connected properly so the transmission shifts pretty early, its hard to tell. I haven't really dogged her out yet as I'm afraid the engine will blow.

Mike Ward

Cables don't cause speedos to read low or fast. Are you sure your car doesn't have a speedo that reads in kilometers per hour?

MotorHead

Yeah Mike he's probably got a Canadian Vette 70mph = ~110 kph

Zorro O

Hey, johnt365.
Nice find on that calculator. Thanks for posting the LINK.

Zorro

droknows

I bought the car in AZ, supposedly he was the only owner and he bought it in AZ. The speedo says MPH at the bottom not KPH. Don't think its a Maple Leaf eh.