Using a tire diameter of 26.02" (Eagle GT stock diameter) there is NO driven gear for a TH400 that will net you where you want to go without a gear ratio adapter. If you use different tires, ensure you know the exact diameter or rolling circumfrence of the tire, usually in inches. You need to convert to revolutions per mile is the idea.
Diameter would use 20,168 and divide that by the diameter to get the revs per mile. 20,168/26.02 = 775.09 revs/mile .If using rolling circumfrence, you'll need to convert to feet per one revolution, then divide that into a mile in feet (5,280). For the Eagle GT 215/65-15, it's 6.812 feet rolling circumference. Thus, 5280/6.812 = 775.14 revs per mile.
Revs per mile x final drive ratio x drive teeth and divide that by 1001 = the number of driven teeth needed. TH400 uses 34-45 teeth. (NOTE: 18 teeth is the typical factory TH400 drive gear, but others are available. Changing it will require some slight tail end invasive work to do on a short shaft TH400.)
Anything you get outside the 34-45 tooth driven gear range would need changing the drive gear or getting an adapter.
This guy here, whoever he is,
😆 thinks he knows what he's doing by installing his drive gear. Actually,
oldsofb does an excellent how-to video on how to do this, as all his vids are excellent. Ironically it's going in an 87 442. He installs a 15 tooth drive gear, and if you used that same 15 tooth drive gear with 3.73 and stock Eagle GTs, you'd need a 43 tooth (p/n 1362196) without an adapter. He's using different tires, and all that, so his setup is likely different than what you're seeking perhaps, so just use the procedure within regardless of which gear set you use.
Now, assuming you have an 18 tooth drive gear and don't want to change it out, you will get 37.5457 gear needed by using a recommended 0.7222 adapter. If you use a 0.7333 adapter, you get 38.1223. Keep in mind, the further you are from a whole number, the more the speedo error may occur.
Red 37 tooth or Blue 38 tooth should get you started in this case. I'd probably go with the 38, but that's up to you. Blue 38 is p/n 1359272. Red 37 is p/n 1359271.
There are two speedo gear housings that go into the side of the trans tail for the driven gear. 34-39 tooth version and a 40-45 tooth version. You would use the 34-39 tooth version with the adapter mentioned. GM p/n 397807. The 40-45 tooth version is GM p/n 397806.
Good luck.
