Ok, I put a new rear in my car today. The new rear has 3.23 gears, the old had 2.73 I believe. I have a 700r4 Trans, 88 T/A, 350 TPI, any ideas on what speedo gears I need? BTW - Speedo stopped working when the new rear went in, not inaccurate, just not working at all, and would that cause a check engine light?

 

Assuming that you have 245/50/16 tires:

You could do:

15T Gray (8622620) Drive
39T Brown (1359273) Driven

OR

Since you probably already have a 17T Drive,
you could do:

17T Red (8640517)
44T Gray (9780470)

If you have the original gears in there, you probably wanna change both anyways. Both seem to be about the same degree of accuracy. I would go with the first combo. In case you want to change gearing in the future again, the 15T Drive gear seems to be more applicable to other combinations.

As for your speedo not working, check the cable and the speedo itself. Speedo's are veru cheap at junkyards. Because of the VSS, I think it can cause a code, but I could be wrong.

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1989 IROC-Z 5.7L