Gear Ratio calculation spreadsheet

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)

what about people that change out the diff from the 3.29 ratio to the 3.05 and go with 3.41 gearing...

Here is a shift point chart for all 3 gear ratio's. 2.93, 3.29, & 3.69's. This is useful when trying to calculate shift points and rpm's.

There are 3 tabs, one for each gear ratio.

Quote:

---

Originally Posted by scoggin

what about people that change out the diff from the 3.29 ratio to the 3.05 and go with 3.41 gearing...

---

Ouch, i'd start with a stock bin and go under the speedo tab. Easiest way to get this to work is to keep checking your vss number and get a 3rd gear ratio of 1:1. This would be a trial and error method since there really isn't a basis that i can figure out. Go for a ride and scan your calculated 3rd gear ratio. Take the value you actually scanned and divide it into 1 to get your percent error. Adjust your diff ratio under the speedo value to change the vss number to equal the value you calculated. Like i mentioned in my original post above:

"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."

This is the most solid way I can think of to dial it in based on those values (3rd gear ratio should always be 1:1 or the computer will throw a wrong gear ratio code which you shouldn't disable)

ok, ill try that when the diff comes in thanks

Wonder if this is a problem: mine reads .984 in third, but bounces to 1.0 at times??? Would I need to adjust it as well?

I really didnt have time to read the whole post but....If your going from stock 2.93's to 3.29's do this...Using your 100% stock gear ratio table from the GTP file change the current gear ratio from 3.29 to 3.69, thats it and nothing more. It will make the calculations automatically to adjust tables properly. If somebody needs to know what to put in if you have 3.69 gears lemme know but i think it is 4.10. doing it this way EVERYTHING works as it shoul. speedometer and everything and also no codes because it works like it should. I promise.

So if that's the case, what would a person use for a 3.43 FDR? How are you making the calculations that determine this number? Just curious... I'm trying to get a better grasp on the concepts at work here.

3.42....or 3.41 depending on your rounding

Quote:

---

Originally Posted by MMGT1

Wonder if this is a problem: mine reads .984 in third, but bounces to 1.0 at times??? Would I need to adjust it as well?

---

Does it throw a code? If it doesn't you're probably fine, it's pretty close.

Quote:

---

Originally Posted by scoggin

3.42....or 3.41 depending on your rounding

---

I'm confused by this...

In the post above, TDCRacing stated that for a change to 3.29 FDR the gear ratio in HPT should be set to 3.69. He also stated that when changing to a 3.69 the ratio should be set to 4.10. Based on this, it would seem that I would input something other than 3.41 or 3.42 for a 3.43 FDR. How are these numbers being determined?

BTW, right now I have 3.43 set as my gear ratio and it isn't working correctly. Shifts were not scaled properly, as I have slow/soft WOT shifts (I have another thread going about this issue). Ideas?

Stock gear ratios are 2.93 in a unmodified transmission right? The FDR on a stock bin file for that same car is 3.29. For example if your going from a 2.93 gear to a 3.29 gear you would change the stock FDR ratio in the bin file from 3.29 to 3.69 and automaticly scale transmission tables.


let me find a more accurate answer for you and i will get back with yall tomorror. I am very busy right now

Type in a gear ratio, hit commit, save file as another name and go for a ride. Scan your calculated ratio and command third gear in the shifter. Should read 1.00 if it doesn't try a different number than what you previously input. 4.10 gets you close for a 3.69 gear but if you scan calculated ratio a 4.11 will get the 3rd gear ratio closer to 1.00.

I'll update the main post to show this.

Thanks I forgot about the calculated gear ratio in third. And yes 4.10 should be good for 3.69's

This is a good thread, when I switched to the 3:69 gears in 2005 ZZP sold me a 34 tooth reluctor and I still couldn't get the speedo working correctly.
The result was that I had to buy a micro controller to calibrate my speedo but the scanner sees the actual difference so if the dash said i was running 65 mph the scanner might say that I'm going 90 mph.

Now that someone has figured this stuff out, I'd like to put the correct reluctor in and cal this thing correctly because its a bummer tunning my shifts with two different data points.

Thanks for the data gents.. :cool:

ok sorry to bring this back up but im new here! I want to make sure my speedo is correct and the last person who did a tune for me has moved away and he changed my settings so i dont know hwat they were stock. I have verified by my RPO codes i have the 2.93 ratio. From what i have gathered here is that NOT to put 2.93 in the gear ratio box under tire size but instead put 3.29? That seems strange to me, but there must be reason that i dont understand.

If you have 2.93's stock then you will put 3.29's in the box...that is what is in the box in the stock pcm anyway. If dave will chime in im sure he'll explain it more.

Quote:

---

Originally Posted by cujo

ok sorry to bring this back up but im new here! I want to make sure my speedo is correct and the last person who did a tune for me has moved away and he changed my settings so i dont know hwat they were stock. I have verified by my RPO codes i have the 2.93 ratio. From what i have gathered here is that NOT to put 2.93 in the gear ratio box under tire size but instead put 3.29? That seems strange to me, but there must be reason that i dont understand.

---

Take the gear ratio that is in your PCM and divide it by the drive sprocket ratio which equals your FDR. Mine is a stock 04 MC so my Calculated gear = 3.29 / Drive Sprocket Ratio = 1.121 equals your FDR. Mine is 2.934879~~~~. If you want to calculate a new Calculated gear it would be the gear * Drive Sproket Ratio = Calculated gear.

Example: I want to put in 3.69 gears in my car. So multiply 3.69 * 1.121 = 4.13649.

HTH

No, you only put the final drive, i.e. 3.05 or 3.29, that is it. I went from 3.69 to a 3.42 and only changed the final drive ratio, the speedo is right on, and everything. if you stay within the same final drive ratio, but change the sprockets, you dont have to change anything. if you change the tire height you will have to update it. that is my exp on it though

when i put 3.42 in it, it was all sorts of messed up, i was going about 50 and it registered 15mph...

I am confused...i put a HD diff from a 06 GTP into my 07 GT. My 07 GT has a gear ratio of 3.05