Hi Scott,
I just replied to your email but wanted to share with others on the gear ratio change what I do as well as what a lot of others use in regards to tuning with HPTuners (this does not apply for DHP users!). I have made several posts with this info before but thought I would bring it back up as it has been a long while.
The stock GTP overall gear ratio is 2.93:1. You actually have a 3.29:1 final drive-differential ratio, but because of the drive/driven sprocket combination which is 37 drive and 33 driven this overdrives the transmission by 1.121:1 resulting in the adverstised 2.93: overall ratio.
The factory tune shows a 3.29 gear ratio and 1.121 sprocket ratio for the supercharged cars (with exception of 04+ compG 3.29 cars). A lot of people get in a mess over this and does not work how you think it would. You cannot simply plug in stock GT 3.29 gear ratio numbers on the entire speed page as it does not work- there are other numbers involved beyond what is on the available tables to play with in HPT so we have to use what is there- and its really simply.
When you are changing the sprockets to 35/35 this gives you a 1.00:1 ratio. You simply multiply the difference by the gear ratio in the tune, so 1.121 X 3.29 which comes out to 3.69. No- this is not your actual gear ratio, its just math that the pcm is looking at and needs to see a number to make things align correctly. A great feature with HPTuners is that it has the option to automatically skew all MPH related tables in the transmission portion of the tune- so falsely entering 3.69 will not only keep the numbers correct looking at input to output speed values, but also will skew all mph tables by the 12% change so you dont have to. It is one simple change that does everything at the same time.
You need to verify your gear ratio is being correctly seen by the pcm and to do this open up your scanner and watch "current gear ratio" and drive the car in 3rd gear. You should see 1.00:1 +/- .02 and then you know it is correct. NEVER turn off code P0730 for incorrect gear ratio!!!! Turning this code off when there really is a ratio problem opens a whole new can of worms and you cannot diagnose the problems if the code is setting but wont report- just turning the code off does not make the pcm ignore it and can cause a lot of side effects for no apparent reason. Leave the code turned on, it wont set if the tune is correct.
Now- whether this has anything to do with your problem at hand I dont know but as you mentioned you should make sure this is correct before going any further. Vibrations under load are typically driveline related or driveline angle related from a lowered car. I have heard similar stories before and ended up being a combination of wheel bearing and worn axle problems cause the vibrations. You just have to make a checklist of what was done when the trans was installed and start ruling things out since the vibration did not exist before. Was anything else replaced during the install? It can be as simple as a different mount or cradle alignment.
