So I just put in a new 700r4 in my 1990 Iroc Z the old transmission had the red plastic driven gear with the red drive gear on the speed sensor. The new transmission has a metal gear with the new sensor. My problem is that when I plugged In the new sensor my speedometer pegs out as soon as I give it gas and then kills the motor I can unplug it and no issues drives and shifts perfect. Is this something to do with the ECM I did a resistance test and the new one reads a higher ohms than my original?

 

https://www.summitracing.com/parts/ado-24202711 may be the "gear" you have.
It has 40 teeth per output revolution times hundreds of revolutions per mile. Well over 20,000 pulses per mile.

The stock VSS with speedometer gears puts out about 4004 pulses per mile with correct gears.

Got the stock stuff? Put it in. Or else, you will need to use a truck DRAC, or tune the EPROM, or use a Dakota Digital SGI-5 to change the signal.

 

Can you just swap the old gears to the new trans?

 

Yes may have to drill a hole for the gear clip but that seems to be the most reasonable fix!