Differential and speedometer

[BCK1]

I have had my differential replaced and now notice that the speedometer is reading too high [ ie 100 kph reads about 120 kph on the speedo] I suspect that the original diff had a 3.9 ratio and the replacement is probably a 4.3. My spider is a 1983 Pininfarina with a 5 speed gearbox. I have read on the forum that if the diff ratio is changed then the speedo drive off of the transmission will need to be changed. Is this a difficult issue to deal with and are the needed parts readily available? Thanks Brian K

[baltobernie]

Salvage speedo drives are readily available. I don't know, however, if anybody has successfully replaced one with the transmission still in the car. While I had a mismatch, I just got in the habit of looking at the tach. 3,000 was 55 mph, etc. and it took me a while to go back to the speedo once I had everything sorted.

I think speedo drives are color-coded with a paint dot, but I don't know the scheme.

So how do you like your new ratio? With 2 litres, that car should really jump off the line.

[BCK1]

Thanks for the reply. As I thought, it might be too big a task to fix while the transmission is still in the car. I can't say that I've noticed too much change but am a little concerned at higher speeds as the RPM is about 4000 at 100 Kph [62.5 Mph] Brian K

[azruss]

replacing the speedo drive is no big deal. the big deal is that each has a different gear to match up with the tranny. this means pulling the tranny and replacing the speedo drive to match up. may be easier to just map the needle placement and have a new speedo face made.

[Exit98]

I just changed out my rear to a 3.90. The speedo is about 4 mph slow at 60. Doesn't bother me I just add a few mph to whatever's showing. Small price to pay for a great improvement.

But when I do drop the tranny for whatever reason, which will happen sooner or later, I'd like to have the right gearset to drop in. I'll have to speak to George about that.

If I go anywhere special I'll have the GPS along so there's always that.