Calibrating speedometer.

[fast_n_low86]

I recently replaced my 3.08 gearing to 3.42 on my 8.5" and since then my speedo is out to lunch when I'm trying to do highway speed (100km/h). Needle is buried when I'm only doing 109km/h.

How could I recalibrate this. Also have my winter tires on and the are 235/75r15. Also will have to adjust for my summer tires which are 235/70r15 rear and 225/55r15 front. Transmission is th350c.

 

[WebMonkey]

you have to change the speedo drive gear.
in the transmission if you're talking about the 2 wheel drive truck in your profile.

 

[fast_n_low86]

Yes. That is the one. I've never done any work in a transmission before so I'm not sure how to go about that or anything

 

[Ricko1966]

Look up something that has the same gear ratio,transmission,and approximately the same size tires order transmission speedometer gears for that,pull the driveshaft and the transmission tail shaft. It is totally self explanatory from there.

 

[idahovette]

Should be able to just change the driven gear and get satisfactory results??......unless it was not correct from the start.

 

[Ricko1966]

Well things just keep getting easier. Now there is this handy calculator that figures everything for you.

TCI Auto Speedometer Gear Calculator - TCI® Auto

Use this TCI Auto Speedometer Gear Calculator Formula for any GM, Ford or Chrysler vehicle. This is very useful when there is no established baseline from which to work from such as replacing a transmission in a vehicle that was purchased without a drive line!

www.tciauto.com

 

[75gmck25]

I’ve never changed one myself, but aren’t the gears color coded so it’s easier to tell which one you have?

 

[Bennyt]

Well things just keep getting easier. Now there is this handy calculator that figures everything for you.

TCI Auto Speedometer Gear Calculator - TCI® Auto

Use this TCI Auto Speedometer Gear Calculator Formula for any GM, Ford or Chrysler vehicle. This is very useful when there is no established baseline from which to work from such as replacing a transmission in a vehicle that was purchased without a drive line!

www.tciauto.com

This is great. Exactly what I've been looking for!

 

[Camar068]

Few links in the post below:

GMSB speedo

LASPEEDOMETERGEAR

 

[Matt69olds]

I’ve never changed one myself, but aren’t the gears color coded so it’s easier to tell which one you have?

They are color coded, but unless they are new gears it’s best to count teeth. They tend to discolor after 30 years in trans fluid.

 

[fast_n_low86]

Yeah. I can only imagine