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Old 08-12-2010, 01:09 AM   #1
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Default Gear Ratio question

Hello everyone. I am planning on installing the HD Differential upgrade with new ring gear to adapt the 4t65e-HD differential to the 4t65e from ZZ performance on my 95 and had a question regarding the gear ratio. The 4t60e currently has a gear ratio of 3.06 and my transmission guy was saying by swapping it out with an HD differential in most cases it may change the ratio. If this is the case, would this be something simple such as a chip tune or will I also have to swap out other components on my transaxle to avoid any issues? I would have thought my transmission guy would have had the answer for me but he was unsure about this one. I had the transmission rebuilt last May and has ran great except that I have had to replace the differential once since the rebuild. Any advice with this would be greatly appreciated.
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Old 08-12-2010, 01:34 AM   #2
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Hmm, generally a gear ratio change is a simple change in the tune. But you have a 95 and that'* a bit different. lets wait for Billboost to chime in. He should know. Or perhaps SGP358 might know.
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Old 08-12-2010, 02:27 AM   #3
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IIRC the HD differential and case from a 4T65E is not compatible with a 4T60E.

The differential itself is a 3.29 ratio. On the 4T65E-HD, the sprockets are 33:37, which combined with a 3.29 differentail, gives you the 2.93 FDR. (33/37*3.29=2.93).
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Old 08-12-2010, 10:35 AM   #4
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I'm not sure on mixing the two transmission parts. I've had them sitting side by side and overall they don't appear to be much different. The 4T60 went through many ring gear and teeth/pitch changes. Make sure that things are correct in all aspects before doing this or it will eat itself up quickly.

The reason your trans guy doesn't know this stuff is because 95% of these guys take the existing trans out and rebuild it back to a new unit. No changing of ratios etc. They can order parts from the tag on the trans/vin and model of vehicle and it'* an easy directly fitting part each time. You are modding and modding info comes from different sources. Dave at Triple Edge Performance 260-437-8912 would be a good source for the info you are looking into.

In the 4T65 HD and non HD you have very few parts that are different. The only differences are the drive and drive sproket tooth count and the differential'*.

4T65 Drive/Driven gears
All non-HD 4T65'* had a drive and driven ratio of 1:1 with 35 teeth on each gear. HD had 1:1.21 or 33/37 tooth counts.

4T65 Differential
Non HD gear ratios available. IIRC 2.86 and 3.05 in the H bodies, GP GT added 3.29 for sportiness. This is a non-hd 3.29.
HD only a 3.29 ratio was available, this is the HD diff.
Each differential has a specific sun gear that drives the planetaries and must be correct based on the gear ratio. The HD diff cover is larger to accomodate the beefier diff.

Beyond the drive/driven and differential/sun gear/diff cover differences, the 4T65 HD and Non-HD transmissions are identical.

Now on to that ring gear. In the 4T65 it'* the first part that is put into an empty casing and must be put in before the rest of the transmission. To swap a ring gear requires disassembling the entire 4T65. IIRC it might be possible to swap the 4T60 w/o pulling the whole trans, I am not familiar with that though. SGP358 would know more. I'm relatively new to working on the transmissions of our cars.
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Old 08-12-2010, 12:31 PM   #5
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could avoid the confusion and grab the required parts from a 96 L67 car.... those were 4T60-E HD, a 1 year deal....

and since the speed sensor reads the driven section of the trans, not the driving section, it won't mess with your speedometer reading, or the PCM speed value as long as the VSS reluctor has the same number of teeth.

different number of teeth would require a little massaging to the road speed constant(to account for more or less teeth)to keep the speedo accurate.

though if you do grab quicker gears than what your PCM is programmed for, your WOT shifts may or may not happen later than they normally would, i can't remember if the 94-95 3800 goes off of RPM AND MPH, or just RPM alone.

simple fix for anyone that can burn chips though.
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Old 08-12-2010, 02:57 PM   #6
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Just a tip, if you want a 3.29 FDR HD transmission, grab a transmission from a buick century (3100) or other 3100 powered car, which has 35:35 sprockets, install the 3.29 HD diff and housing, and done!

I picked my transmission up off ebay for $225 shipped with 86k miles.
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Old 08-12-2010, 06:36 PM   #7
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Thanks everyone for the info. I looked into some more things today and I am going to have Ryan at Sinister Performance burn a new chip for me. By researching my RPO codes as said above my car does have a 3.06 ratio currently. ZZ Performance confirmed today that my new HD differential has a 3.29 ratio. So the next question is based off of what XTREMEREVOLUTION said, do I have Ryan put my gear ratio at the 3.29 or at 2.93? Just want to make sure I have the correct ratio to avoid issues. Thanks again everyone for the information, greatly appreciated.
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Old 08-12-2010, 06:46 PM   #8
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Quote:
Originally Posted by bigT View Post
Thanks everyone for the info. I looked into some more things today and I am going to have Ryan at Sinister Performance burn a new chip for me. By researching my RPO codes as said above my car does have a 3.06 ratio currently. ZZ Performance confirmed today that my new HD differential has a 3.29 ratio. So the next question is based off of what XTREMEREVOLUTION said, do I have Ryan put my gear ratio at the 3.29 or at 2.93? Just want to make sure I have the correct ratio to avoid issues. Thanks again everyone for the information, greatly appreciated.
You didn't read my post correctly.

The 4T65E-HD transmission has an HD differentail that has a 3.29 ratio. The transmission'* chain sprockets are 33:37, which when you calculate it based on the calculation I made above, gives you a 2.93 final drive ratio for the entire transmission.

If you have a non-HD 4T65E, you'll have a 35:35 sprocket ratio, and a 3.0x differential, for a 3.0x final drive ratio.

Now, since we know that the HD differential is a 3.29 ratio, you can then install that in a non-HD 4T65E and basically get a HD transmission with 3.29 final drive ratio.
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Old 08-12-2010, 06:51 PM   #9
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^knows what he'* talking about!

we only went through how many pages trying to figure out WTF to do about your regal...
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Old 08-12-2010, 06:53 PM   #10
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Ok, I understand now, I guess I didn't read it correctly. Thanks for the clarification
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