Speedo calibration in PROM or transaxle?

[Oldsmagnet]

Hey all, I'm a little new to the W-body world (been G-body nut for years) I've had my '90 Cutlass for a little over a year (pics and storyline are here: http://oldsmagnet.oldsgmail.com/90cut.htm )

 

Previous owner replaced the tranny at some point, a used piece (I'm presuming from a Grand Prix, but I've not actually dug down to the ID tag to find out for sure) Problem is, now, the speedo is ~way~ off. Generally no big deal, out of 50+ cars I've maybe got 5 or 6 accurate speedos in the bunch :? but this one is different, since the thing governs out around 90 at best (PCM and speedo claim it's 114, or thereabouts) Speedo reads fast by a factor of 1.28.

 

Are the transaxles supposed to be calibrated to x#pulses per mile (60 comes to mind) regardless of final drive? And the overall calibration for speed performed in the PCM/PROM? Or has my speed sensor in the tranny gone south? I suppose the GP or whatever donated the tranny had larger tires, but I don't think all the error can be attributed to that alone.

 

I'm waiting for a 'nice' day to pull numbers off the transtag and RPO sticker, I think I've got enough service manual info to determine what final drive ratio it does have, and used to have, but that's as far as I'll be able to get on my own.

 

I'm told this car is pretty quick for a 3.1L, I'm inclined to believe the previous owner got an accidental performance boost (as did I) by the lower final drive. The smaller tires should help too (if they ~are~ in fact smaller than the GP/donor's) Now if it were just a supercharged 3.8, I wouldn't ~mind~ the errors, but

 

Anyway, anyone have ideas/suggestions for making the speedo accurate? It's a 4spd auto, by the way.

 

Thanks in advance,