Final Drive Misinformation?

[Confusingboat]

So I've just been checking out some specs on new-cars.com and it says that the 04+ NA has a 3.05 final drive ratio, however everything I've read from here (GPF) suggests the 04+ NA has a 3.29 final drive ratio. Which is correct?

[matt5112]

It should be 3.29's.

I didn't think they ever gave the GP any other differential ratio other than 3.29 which means 3.05 wasn't possible.

[BillBoost37]

It should be 3.29's.
I didn't think they ever gave the GP any other differential ratio other than 3.29 which means 3.05 wasn't possible.

Well.... other body styles got 2.84 or 3.05 in the NA models. The 4T65E can be found in those ratios, typically not in a GP though.

[01gpgt01]

Most of the buick N/a line up got 3.05's Pontiacs 3.29 N/a

[BillBoost37]

Pontiacs? Bonneville didn't get a 3.29... just saying

[Confusingboat]

Well it has the 97-03 NA listed as 3.29, and all the other years/trims listed correctly. So maybe it does have the 3.05? Any surefire way to tell?

[matt5112]

what speed does your car shift at at the top of first?

Better way is get up to 50 MPH, let the car shift into fourth and let the TC lock up then look at the engine speed.

you can then calculate the final drive based on that information.

[Confusingboat]

~50mph.

[matt5112]

i'd venture to say 3.05s, but try the other method i edited into my post.

More accurate.

first one is just a guessing game really.

[Confusingboat]

How exactly does one calculate this? Btw at 70mph in OD the needle is right on 2k RPM fwiw.

[matt5112]

Potential Engine Speed (RPM) Calculator

[matt5112]

3.29's 2023 rpm at 70 mph

3.05's 1888 rpm at 70 mph.

have a scan gauge? Not sure how accurate your tach is.

[Confusingboat]

I have Torque for Android. I'll check it out before I go to work, but assuming the tach is accurate I have 3.29's right?

[Trannyman95]

Some 04+ GPs did come factory with 3.05 final drives and seemed like more of the lessor optioned cars came this this. 3.29 was an option as well. The ID tag is the easiest thing to check to verify what the trans was built with from the factory.

[matt5112]

I have Torque for Android. I'll check it out before I go to work, but assuming the tach is accurate I have 3.29's right?

yup

ID tag works as well.

[Confusingboat]

Where does one find this ID tag? And I have a GT2 with options so it probably is the 3.29.

[matt5112]

its on the case.

can't remember where.

probably on the back or side.

[Trannyman95]

The ID tag is on the back of the trans case near the passanger side. This can only be seen from the bottom of the car and you can look up near the steering rack area and see it. If yours is a GT2 then it likely does have the 3.29 ratio. The few that I have rebuilt for the NA cars that were 3.05 ratio were GT1 cars.

[Confusingboat]

Forgot to post back, but my tach is dead on and the top of 1st gear is moreso around 48mph than 50. I'm gonna go with 3.29 on this one. Thanks for all the help in understanding this one guys.

[Overkill]

I haven't been in an 04+ Grand Prix N/A that didn't have a 3.05 diff.

This is an 04 GT with one of my PCMs. Note the GTP-like MPH shift points. YouTube - GRAND PRIX GT 0-60 mph 7.2 seconds 2004