I have recently had some rearend work done on the '78 Camaro with new Truetrac posi and 3.90 gears (changed the 2.41s out) and the car is a completely different animal now....Including the speedo. Depending on how fast I'm going the speedo is on average 40mph off. A little less if going slower (around 30mph off) and much more if I'm going faster. Still not sure if I'll keep the TH350 after the new engine gets installed but would really like to fix the speedo gear if it's not too big of a project? Is this more like a trail and error fix? Any insight is appreciated as always.

 

Here's a formula to figure where to go: Speedometer Calibration Program - Method 1
The speedo driven gears are readily available in 17-22 teeth. Figuring your error factor: 3.90 divided by 2.41 = 1.618, your difference is too great to change only the driven speedo gear. If the rearend gear difference was more slight, you would multiply the 1.618 by the old driven gear to give you the new number of teeth you need. More teeth on the driven gear will slow down the speedo, but that won't be enough for yours, you'll need to change both the drive and driven gear.

 

Thanks for the info. Haven't ever messed with the drive/driven gears before. Can't be too hard to get them out of the trans, just wasn't sure how to go about doing it. Do you simply unscrew the drive gear from the tail shaft and pull the cable/driven gear out together?

 

It's a little more involved than that. You need to remove the driveshaft, and then the tail housing on the trans. The drive gear is held onto the output shaft with a spring clip. Those clips sometimes break, so best to buy a new one with the gears.
Another option is to put on a speedo ratio adapter that goes in between the speedo cable and the driven gear housing. It's a lot easier than taking everything apart, but can cost more.