I swapped a 4r70w out of an '03 Mustang into my T-bird. The bird has 3.73 rear gears.

Before the tranny swap, I had a speedo gear that worked fine with the 3.73's. Since the tranny swap, we've tried a couple of different speedo gears, but the speedometer is mis-calibrated ... it reads faster than the car is actually traveling.

I've been told by one source that the 4r70w's in '03 Mustangs have a different tailshaft than the T-bird had - one has a 7 spline shaft, one has an 8 spline shaft and to be able to get the speedometer properly calibrated, I need to either swap tailshafts, or use the FRPP auto speedcal.

Another source says that it isn't the tailshaft spline gears that are different, it is a gear inside the transmission that has to be changed.

Can someone give me the straight poop on what the differences are between the T-bird 4r70w and the later model 4r's that cause the speedometer calibration problem?

Thanks.

-mike

Nevermind, I got it figured out... even got the part number for the 7 tooth tailshaft I need.

you should be able to run a black 23 tooth VSS gear with the 8-tooth tailshaft.

Or for the 7-tooth you would need the Purple 21 tooth VSS gear..

Thanks .. I was told the same thing about the purple gear and the 3.73 gears in the T-bird.

The word I heard from many sources is that the 23 tooth VVS gears tend to self destruct.

Other trans issues force me to have to drop the trans and take it apart, so while I have it out, I'll swap in a 7 tooth tailshaft and make it easier on everyone.

A couple of weeks ago, I lost 3rd and 4th gears. Trans shifts ok through 1st and 2nd, then neutralizes when it shifts to 3rd. I suspect a broken intermediate stub shaft, but won't know 'til it gets torn down, while it's apart I'll swap in the other tailshaft.

thx
-mike