Original Trans?

[H/O84]

Running a set of ancient 215 65 15s eagle gt tires from the 80s. Once it's painted and roadworthy I'll get some new tires for it.

 

[L67ss]

Yeah that's stock size

 

[H/O84]

Could I'd be that a gear is missing a tooth?

 

[L67ss]

Itd have to be in the odometer not the trans

 

[H/O84]

Any idea what in the odometer is wrong

 

[H/O84]

It definitely has a green gear in it and according to my 84 service manual that's not what the factory used. Could it be wrong gearing?

 

[L67ss]

If it was the trans causing the odometer issues the speedo would be a lot further off. *ss in showing 60 when youre doing 30. I'd say its issues with the cluster

 

[454GrandPrix]

Important point nobody has mentioned:

The car is an 84 Hurst olds so it has a 3.73 gear and a 200-4r.

Have you verified this? Assumptions don't count. Presuming you did not install that ring & pinion yourself, have you actually measured to see what ratio you have in there?

 

[H/O84]

Yes I changed the fluid a few weeks ago and checked the gear it is a 3.73.

 

[84 W40]

L67ss is correct about the speedo being off from the factory. I can remember pulling a brand new cluster out of a new buick GN to have it Certified before he took delivery, the owner was a state cop. That speedo was off by 8 mph. Your trans should be OZ for 84, if you look at the drivers side pan rail on your trans there is a small pad with stamped numbers on it. Those numbers should match your vin numbers, last six. With 3.73 rear you should have a 30 tooth light blue speedo gear and on your governor 10 tooth green gear. What happens to a speedo [cable driven] over time the spring thats connected to the needle becomes week and or the magnet that rotates around the cup that creates a magnetic field gets week and will throw off your MPH. To fix your MPH you have two options have the speedo calibrated or find a 31 tooth gear that should slow it down by 3 MPH.