After doing some searching here and coming up with a lot good but old info on the trans speedo gear needed to be closer to accurate.
Specs on effective parts
700R4 original to 1991 LO3 305TBI with a 120mph speedo, car originally came with a 3.08 up diff rear end.
current rear Borg Warner 9 bolt with a 3.27 gear
Rear tire size 275/40/17 25.7"-inch-tall tire which is the same height as a 245/50/16 OEM Trans AM and Z-28 tires.

Help me understand Rear gear change speedo-gear correction and what gears I need
Thanks

 

Check out the threads that are stickies at the top of page in this Transmissions forum. The charts for speedo gear selection aren't necessarily accurate though and may only get you in the right ballpark. I recommend you buy a pack of different gears and experiment until you get the speedo accurate.

 

https://www.thirdgen.org/forums/elec...correctly.html post #4

DO NOT try to use the tire "trade size" for "calculating" tire diameter!!! Pull a tape on it.

I would also add, your 9-bolt DOES NOT have 3.23 gears, WHATEVER it has. They are NOT that, they are SOMETHING ELSE, guaranteed.

While you're changing out your rear end grease that obviously needs it because you obviously haven't, LOOK AT the gears. More likely they're either 2.77 or 3.08, if they came in that car; or 3.27 if they came from somewhere else.

 

Sofa, Always the word smith
It has been quite awhile since I’ve looked at them and your right, they’re 3.27, going off memory after the cars been in the garage for 15 years and haven’t looked at the gears in 8-10 years. Rear came from a 88-89 Z-28….. the gear oil is fresh and new 🤔

 

Sofa, your post mentions using center line of wheel and tire?
The brand new never been on the road 275/40/17’s are an overall height of 25.3750 (3/8”)

 

That's a pretty short tire for a thirdgen. A 45 series would probably be more ideal...

 

A 275/40/17 and factory size 245/50/16 are both supposed to be 25.7” tall at least that’s what the trade numbers say.

 

Ignore the "trade size". It is, at best, an approximation. Your car's weight, the pressure you run in your tires, the temperature that they reach while driving, etc., will ALL affect the ACTUAL size that the tire presents to the outside world. What you need to be concerned about is the RADIUS from the center of the wheel to the Earth, NOT the "diameter" shown in the "catalog".

Use a tape measure on YOUR tires when installed on YOUR car.

 

275/40-17 is equivalent height to stock for all practical purposes. There is variation from brand to brand, model to model, worn versus new. That's all noise in life, just ignore it. Speedometer is not accurate through the range anyway. Best you can do is choose a speed you want to know is accurate and the rest of the range is what it is.

Why even bother with tape measures? You have a driving car so get straight to reality and go for a drive at the speed you want to be accurate and calculate the percent change needed in speedo reading. Snag the gear ratios that get you that percent change of speedo reading. (You have to know your current speedo gear set to make that calculation.)

I calibrate my speedo with a laptop but it's the same concept whether using keyboard or gear ratios. I like my speedo to be accurate at 60 mph so I drive at 60 mph by GPS and read my speedo. If speedo is reading 70 mph, then percent change needed to correct speedo is, [(60 GPS) - (70 speedo)] / (70 speedo) = (-14.3%)

 

Going through and trying to remember things that I did put some thought into years before life got in the way, you bringing up using the actual tire and not the listed size reminded me of posts, some of yours, that I’ve read in the past. As I said the overall height unmounted is 25.3750”

 

Cars a long term project that is not currently on the road yet, very soon though. Knowing that I’m going from a 3.08 gear to 3.27 gear I knew that the speedo gear will need to be changed to getting it ball park close.
I’m my reading it seems that a 3.08 gear would have either a
3.08:1 41 tooth driven 17 tooth drive
3.08:1 36 tooth driven 15 tooth drive
How do I make the determination on a 17 or 15?

 

using the calculation list here,
Sticky: figuring speedometer - Third Generation F-Body Message Boards
20168.0672268 divided by tire diameter, then that times the number of teeth on the speedometer drive gear, then that times the axle ratio, then divide that by 1000
25.3750 tire diameter
17 driven gear
3.27 axle ratio
1000
equals 44.1829 driven gear

20168.0672268 divided by tire diameter,
25.3750 tire diameter
15 driven gear
3.27 axle ratio
1000
equals 38.9849 driven gear

Does sound right a 44 or a 38 driven gear?

 

25.375" (if that is the actual size) is not 25.7". And 25.7 is still a little short considering the size of the wheel arches, and the fact that the early thirdgen Trans Ams & Z-28s had 26" tires...

 

Digging deeper into this, according to the tech data here the 91 LO3 Firebird with 700R4 would have come with a 2.73 rear gear ratio.
1991 Pontiac Firebird - ThirdGen.Org
Which means I currently have 41 tooth driven with 19 tooth drive speedo gears

Can a 44 or 38 tooth driven gear be used with a 19 tooth drive gear?
I read somewhere here that there is a 35 tooth driven gear?

 

Is it not really easy to just remove the driven gear and count the teeth? And then look at the color of the drive gear to figure out which one it is? That's a real question, I have never had a 700R4 myself.

 

Do you have the SPID label? The car's original information is on there. Drive and driven gears are usually D codes, and axle ratio is usually a G code. And what were the original wheels and tires? 15" wheels came with 26" tires, and 16" wheels came with 25.7" tires. With all of that information, at least, you'll know where to begin. If the car came with a 2.73 axle ratio, then it probably came with a 9-tooth drive gear and a 19-tooth driven gear.

Here's the sticky about the drive/driven gears.
T-5 Drive/Driven Gear Info--Please Contribute - Third Generation F-Body Message Boards

And here's a little plug and play calculator to help figure out what you'll need for your 3.27 axle. To use one of your existing gears, then combos of either a 9/23 or 7/19 should do. For all new gears, then a 7/18 would be the combo for a 3.27 axle.
Speedometer Calibration Program - Method 1 (bgsoflex.com)

 

If your only replacing the driven gear that can be removed with the speed sensor but if your replacing the drive gear the tail shaft needs to be disassembled.

 

SPID? Is the sticker with the RPO codes
Car originally came with 15” wheels

 

Yes, the label with the RPO codes.