PMARTINEZ92126

Ok I researched myself into confusion. I have an 86 automatic corvette. I had the rear diff changed to 3.54 gears along with 17 inch rims with 315 tires on the rear. In doing research I was told I needed to change out the drive gear to a 15 tooth gray gear and a 41 driven gear yellow. I was told several things by different shops. FIrst thing I needed to have was the DRAC chip reprogrammed because changing out the gears wont help. Is this the same as the prom chip I sent in to get the specs changed? 2nd I was told to change out the gears only, but when I brought him the driven gear he said that the gear I needed isn't available. I just had the transmission rebuilt and have it installed partially in the drive way. I have the drive shaft out so I figured now would be a good time to try and fix it. Any ideas to how to fix this or should I just put in back in and deal with the speed being off.

JrRifleCoach

Ya know the speedo calibration is based on the number of tire revolutions per mile.
So simply changing the gears on a 86 is your answer.
Most tire mfgrs have RPM info in the tire spec chart.
Search around you'll find many threads that will help decide the gears you'll need.

Dt86

Here's a chart with all of the different gears,axles and tire sizes. http://www.corvetteforum.net/c4/vett...Gearshtm.shtml

I just had to hunt down a 42 tooth driven gear and 308 VSS. They are getting hard to find.

bjankuski

The easiest thing to do is by an electronic speedometer calibration box for Dakota Digital (I believe it is called the SG-5), costs less then $100, wire it in program it and forget about it.

86C4Z51

Put a GPS in it?

rrt898

I changed my rear to a 3.54 also in my 86. I also changed both gears when I rebuilt the trans. I used this site to order my gears http://www.transmissioncenter.net/sp...n_______va.htm.

SouthCoaster

Go here.

http://webpages.charter.net/khasting...peedocalc.html

It's so accurate it'll even factor in your tire squish from the weight of the car.