Just changed out the 3.27's for MK VIII housing with SC freshened T-loc and 3.73's with 245/45-18's on my '90 XR7. Read up on the speedo gears and purchased 23 tooth from Late model Restoration for mustangs. There was a 23 tooth driven gear for 7 tooth or 8 tooth drive gear (2 different gears ava.). According to info from TCCoA we have an 8 tooth drive gear, so I purchased the recommended gear according to their calculator.
The speedo appeared to be at least close for the first 1/2 a dozen short tests. After about 30 miles it suddenly jumped to being low by around 20-25 MPH at highway speed. It shows me at 45 mph @ 2200 rpm, and I am running right with traffic, so I am sure I am at least 60-65. In the recent past the speedo DID act very strange a couple times, and I THINK it may point to my problem. sitting at idle in park, the speedo would suddenly jump up and read some speed, any where from 20 to 65 at various times.
So can I get verification we DO have an 8 tooth drive gear? I understand the wrong gear will blow the teeth off the driven gear in a hurry. The short period of correct operation makes me wonder about this. The calculator at LMR also seemed to assume a 7 tooth gear for MUSTANG AOD's. The TCCoA article stated we have an 8 tooth, and I do trust that info, BUT...
My next thought is that in disturbing the old electronic speedo sending unit it may have suddenly decided to throw in the towel and is sending a low output. In research here I have also seen others with an actual speedo head unit problem. The odometer actually does still work on this unit, if that means anything. B4 I start just changing out parts is there a quick way to pinpoint the head or sending unit as the problem?
Thanks for any help or input
Adam
The speedo appeared to be at least close for the first 1/2 a dozen short tests. After about 30 miles it suddenly jumped to being low by around 20-25 MPH at highway speed. It shows me at 45 mph @ 2200 rpm, and I am running right with traffic, so I am sure I am at least 60-65. In the recent past the speedo DID act very strange a couple times, and I THINK it may point to my problem. sitting at idle in park, the speedo would suddenly jump up and read some speed, any where from 20 to 65 at various times.
So can I get verification we DO have an 8 tooth drive gear? I understand the wrong gear will blow the teeth off the driven gear in a hurry. The short period of correct operation makes me wonder about this. The calculator at LMR also seemed to assume a 7 tooth gear for MUSTANG AOD's. The TCCoA article stated we have an 8 tooth, and I do trust that info, BUT...
My next thought is that in disturbing the old electronic speedo sending unit it may have suddenly decided to throw in the towel and is sending a low output. In research here I have also seen others with an actual speedo head unit problem. The odometer actually does still work on this unit, if that means anything. B4 I start just changing out parts is there a quick way to pinpoint the head or sending unit as the problem?
Thanks for any help or input
Adam
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