I have a big problem that I'm having trouble getting through about this thread.

OP has a 4T60 and is talking about putting a 4T65 HD diff in it. I know that GM changed pitch etc etc many times in the 4T60. Has anyone confirmed that the OP'* 4T60 will accept a 98+ 4T65 HD diff at the ring gear? The output shaft is also different and the passenger axle will need to be shorter to accomodate the longer diff.

Just sayin ...; we are talking about mixing two transmissions and I'm not 100% sure we are all clear on that point.

I seem to remember SGP358 saying they are close, but internal parts do not mix well. I have 3 4t65e sitting on the floor here, but not one 4t60, so I can't even test it out. I know you Bill, have 5 or 6 4t65e sitting there.

My impression was that it was agreed upon that the parts would not interchange. I was simply referring to 4T65E options. They are after all not all that expensive and you can swap in the HD differential and needed parts yourself.

If you do take that route, you will of course have to tune the car to accept the new final drive ratio.

everyone should keep in mind that the stock PCM will NOT control a 65E as intended... the big issue with that is the pressure control solenoid, which the equivalent is a vacuum modulator, something a 60 has, that a 65 doesn't. you can't just drive around without the PCS doing it'* job, you're trans won't last long...

which is why i suggested the HD diff from the one year only 4T60E HD. he wouldn't have to factor in the cost of OBD2 and deal with that until later if neccesary.

I was under the impression that there was a 60HD in nearly every year from 92-96, no?

from the documents i have, it seems it was 96 L67 C, H and G-Bodies only....

the doc also states: "3.05 and 3.29 ratios are "fine pitch" final drives. Sun gears, internal ring gears, and pinion gears are NOT interchangeable with other final drives."

so i'm not sure what kind of wrench that throws into the works...

If 3.05 and 3.29 are both the same pitch/tooth then I think that'* a good thing. The question then comes up as to what was 95 in relation to the 96.

Maybe that'* why the earlier 60'* always blew their diff/diff covers when put behind an SC.

Mike..in the past did you just swap diffs and sungears or did you also change pinion? On the 65 the pinion isn't easily swappable, I don't have info on the 60.

sorry if i don't know the fullsize platforms too well, but here goes:

in 95, the only 60Es to get fine pitch drives were these:

quad4 n-body
3100 l/n body
3800 G-body(*/C and N/A)

everything else had the normal drives.

Thanks again for all the help everyone. One more question regarding the speed sensor. I noticed today that the old differential has 31 teeth for the speed sensor to sense and the new differential only has 30. Will this effect my speedometer or does the ECM automatically correct itself? If not, is this as simple as flashing the new specs on the chip or will this require a different speed sensor?

a tune could fix that in about 10 seconds...

although, let'* put it this way: with one tooth difference(31/30), your PCM will read by 3.33% too slow, and your instrument cluster will reflect this as well. not a huge number. hell, tire sizes can make up for that.