For the FWD 3800 guys (4t65E transmission)
I wrote this spreadsheet to calculate all the necessary values for doing a gear swap. It has worked for me so I’m thinking some others may be interested; possibly maybe make this a sticky. I know this question has come up several times and nobody really has an answer.
I'm using 3.69's and a larger tire than stock so note that if you are using it. I've tried to make it as simple as possible, enter the highlighted values and it will calculate the correct input. All values are changed under the speedo tab in HPT.
Also, to make this work correctly you will need to scale your shift tables which can be done very easily. Under the speedo tab click the button which says "manual scale" and scale them by the necessary value (i.e., 2.93/3.69=.794, 2.93 stock and I went to 3.69). Click commit and bang done, this will get them very close, they will require some fine tuning to dial them in.
Now, you may get a wrong gear ratio code set by doing this DO NOT DISABLE the CODE, this can potentially cause damage to the transmission by commanding excessive and unnecessary pressure (from what I’ve read in GM service manuals and many transmission guys have mentioned this).
Edit:
Here's an update to this, in order to fix the car so it won't throw a wrong gear ratio code you have to change the VSS pulses per rev trans. so that in 3rd gear the calculated ratio reads 1.00. I test drove the car after making the necessary changes and then fixed it.
So mine was 9.118 (from my spreadsheet) and my calculated ratio was reading 1.25. Therefore, 1.00/1.25=0.8, 9.118 x 0.8=7.29 for my calculated VSS pulses per rev. trans. The other thing i've found is leave the trans revs per mile stock, no matter what, even using my spreadsheet to change it to work with the tires it screws up the tcc calcs and caused me a year of aggravation trying to figure it out.
In essence you will change your sprocket ratio and tweak VSS pulses per rev trans, then back calculate your VSS pulses per mile (if you want to).
Alternate way of doing this:
Leaving all the values in the speedo calc's stock.
Under the speedo tab is the gear ratio conversion feature. Takes a slightly different approach than say changing the ratio in an Fbody.
On the top you see tire size and gear ratio. Type in your stock tire size (should be listed to the left, carry it over). Then type in your ratio as follows:
If you're using a 3.29 sprocket ratio type in 3.69 commit.
If you're using a 3.69 sprocket ratio type in 4.11 commt.
Save the file, flash it to the car and go for a ride.
These should get it very close, may take some fine tuning to dial in the shifts. A good way to check is by scanning your 3rd gear calculated ratio. Go for a ride, command 3rd gear using the shifter and see what it reads, if it reads anything but 1.00 you'll throw a wrong gear ratio code and should adjust from there (this shouldn't be the case however unless you changed something else)