I have been using an 85mph cluster since I got my car a few months back. It had always read too fast: When I am doing 30, it says I'm doing 40. When I am actually doing 60, the speedo says I am doing 80. As speed increases, so does the gap between actual speed and read speed. Also, last night I did a swap for a 145 cluster with white overlays (went in perfectly) but now the speedo needle wiggles at constant speed to +/- 5mph (loose connection at the cluster maybe?) I don't know what gears are in the rear end and I am using stock IROC rims. Any help would be appreciated.

 

You need to figure out what rear end gears you have and then get the correct speedo gears in the transmission to make your speedo read correctly.

What RPM is the engine turning while you're doing 60 MPH on the freeway in 4th gear?

 

The following may provide some ideas:

https://www.thirdgen.org/forums/inte...eedo-woes.html

Here's how to determine what ratio the rear is (jerk the diff cover and look): numbers are stamped on the ring and pinion:

41 : 15 = 2.73
41 : 14 = 2.93
40 : 13 = 3.08
42 : 13 = 3.23
41 : 12 = 3.42
41 : 11 = 3.73

JamesC

 

Thanks very much guys, will update in a bit.

 

Stock tach says at 60mph (actual speed) in 4th gear the rpm's are at 2400rpm.