Speedo gear ???

[edfiero1]

New owner of an 84 T-type. My Speedometer is off by around 25%. At 20mph it reads about 5 mph fast, but up over 60 its reading about 15mph fast.

The rear has 255 60 15 Tires on it, so I think that should make me read about 3-4% LOW, right??
I don't have a scanner yet, so not sure what the ECU is reading.

Is there any kind of recalibration that can be done on these speed-o's or is what I need a new speed-o gear for the trans? Does this mean that someone has changed the rear end gear in this car? How would I check that out?
Thanks

 

[mikestertwo]

Could be any reason it is off. Could be someone changed the gears in the rear. Could be the transmission is not original and it has different governor gear/speedo gears in it. What is the transmission code? The code tag is near the tail of the transmission on the right side. First find out what you have. The calibration can be changed by changing the speedo gear in the trans or with an external gear box.

 

[earlbrown]

The only way to know is to get under the car and look. Finding out what speedo drive gear you have will require removing the trans pan.


When I first got my 87 the speedo read slow. Turns out I had a red or green drive gear and I needed an orange one to be right.

It was a pretty simple swap once I figured it out.


At 60 (actual 60, not off your speedo) in 4th with the TC locked how many RPMs are you turning?


and just for grins, how many RPMs are you spinning at an indicated 60?

 

[edfiero1]

The only way to know is to get under the car and look. Finding out what speedo drive gear you have will require removing the trans pan.


When I first got my 87 the speedo read slow. Turns out I had a red or green drive gear and I needed an orange one to be right.

It was a pretty simple swap once I figured it out.


At 60 (actual 60, not off your speedo) in 4th with the TC locked how many RPMs are you turning?


and just for grins, how many RPMs are you spinning at an indicated 60?

I will have to get a powerlogger or something to figure all this out. The Tach is not accurate either. At idle it reads about 3000, and the engine is obviously not turning that fast.
I was hoping I could count the revolutions of the tires or something to figure out if the rearend was changed. Guess not.

 

[mikestertwo]

I was hoping I could count the revolutions of the tires or something to figure out if the rearend was changed. Guess not.

You can. Jack the back of the car up so the wheels are off the ground. Put the transmission in neutral. If the car has a posi rear in it, turn one of the rear tires exactly one revolution (the opposite wheel will turn also) while counting the number of revolutions on the drive shaft. If the drive shaft turns just under 3-1/2 times you have a 3.42 rear. If it turns approximately 4 times you have a 4.11 and so on. If you have an open rear you can turn one side while the other side is stationary and do the same thing. Rotate the one wheel exactly 1 turn, count the number of revolutions on the drive shaft and then multiply it by two.

 

[ScottHeidinger]

Like others have said the rear ends in these cars are seldom changed. The transmissions are much more likely to have been replaced and the speedo gears are probably not the right ones. The easy fix is the external gear box but the cheapest way is probably changing the speedo gear in the trans. It's been a while but there are different colored speedo drive gears based on rear end gearing. I found a chart online before that detailed what color to use to adjust for different tire size or rear end gearing. First step is figuring out what's in yours unless you opt for the more expensive route.

 

[GNRick]

When my speedometer was off I looked at the speedo gears and found the teeth ground down. I think it was the blue gear. They are made out of plastic. I replaced the gear and all was right in turbo land again.