I have a 1991 Camaro RS, i switched to 3.73 gears with positraction, How can I recalibrate the speedometer?
The red came out of the trans the blue didn't fit. Any help!?

 

The red drive gear + red driven gear corresponds to 17 tooth drive, 17t driven, and is correct for 2.73 rear gear and 25.6" tall tires.

For 3.73 gear and the same/stock 25.6" tire, you need 15 tooth drive gear (gray) and 44t driven gear (dark gray). You will need to pull the trans tail housing to swap the drive gear.

The blue gear (38t) you have will work with the gray(15t) drive gear for 3.23 diff gears.

This is according to TCI's calculator and gear charts.
The second section is: Speedometer Gear Calculator
And look for the available gear for the 700R4.

Summit lists an olive green 15t drive and gray 44t driven gear.

 

I have both of those you listed and they don't fit

 

You have an electric signal from the VSS.

You can just get a ratio adapter box. You'll want to convert what's coming from the VSS and convert it to a slower signal before it reaches the VSS buffer (TBI models w/ elec. speedo) or before the ECM (90-92 TPI models)

Speedo gears are defunct and tough to find. No reason to spend $40 on plastic when a ratio box can be had for under $ 100

 

From where

 

I might be wrong- but it seems like the blue gear is made for a cable operated speedo sending unit, while your car uses the 2nd design electric sending unit. ( electric gears and cable gears are NOT the same !! )

The actual electric Speedo sending unit will probably also need to be changed as well as the gear on it to change from 2.77 to 3.73. The speedo sending unit is ( usually ) made for speedo gears with a specific tooth count. This is true at least for all 88 and earlier auto speedo sending units. Example: This speedo sending was used in an 88 Firebird



It will only hold driven gears from 40-45 tooth,.. meaning it can't be used for a 2.77 gear that requires a 39 tooth driven gear. I'm pretty sure the 90-92 speedo sensors are made the same way.

Like Scott said; Rather than trying to figure out / find / and pay for the GM parts needed to correct the speedo reading,.... you could simply re-install all the factory original parts and buy yourself an $85 signal converter from Dakota Digital and avoid dealing with any changes to the stock set-up. Would probably cost you less then swapping to GM original stuff too and if you ever change rears or gears again,.... you can simply re-calibrate.