Not sure what exactly to start with so thats y i'm on here, for all you good fellows help. I've got an 85 iroc with a 355 and a 350turbo tranny. My problem is i'm pretty sure the car only had a 85 mph speedo stock. Someones put a 145mph speedo in it. How in the heck do i calibrate this to work together so my speedo is right? what parts do i need? If i've left something out that u need to know then please let me know.

 

A GM car with correct speedo gears will not need them changed with a different, working speedometer.

60mph should be 1001 rpm of the speedo cable. Only a change of rear tire size or differential ratio would require a change of the speedo gears. What are those 2 items in your car?

 

the tires are the correct 245/50-16. And the ratio is again the correct 3.42. Aint the speedo gear in the tranny? Which has been swapped out cause it's supposed to have the 700r4 in it but its got the turbo350. Plus like i said, some one swapped the speedo out too. I think if it still had the 85mph it may be right on. 55 is alittle further than the half way point on the speedometer. Sorry, I'm just confused trying to figure this out

 

If all the car's components--wheels/tires, rear end, and trans--are original to the car, you can swap any speedo head (85, 110, 140) in that you desire, and the speedo will read correctly.

If you swap in a rear end with a different gear ratio than the original or if you swap in a different transmission that doesn't match the original gear ratio of the rear end, you'll have to swap in the correct drive and/or driven gears, which are in the tail shaft of the trans, to make the speedo read correctly.

JamesC

 

So since the tranny has been changed out from the 700r4 to the turbo350 Then i need to find a speedo gear from a turbo350 that has 3.42 gears in the rear? Just makin sure i understand ya completely.

Or can it be from any tranny with 3.42 gears in the rear?

 

Yes. Check the following for more info:

http://www.transmissioncenter.net/sp...____va.htm#350

JamesC

 

Yes. An 83/84 Monte Carlo SS TH350-C would have correct speedo gear teeth.