Is is safe to say it's high speed driving on the freeway that is the *reason* these boxes wear out? Or is it high-torque situations that wear them down which then makes them likely to fail once they are on the freeway?I had two catastrophic T5 failures. In both instances, two people in the Scrambler, probably 400 pounds of stuff in the back. Both times happened after sustained interstate driving at around 75mph, probably 3-4 hours running time.
Both times, it just went boom. Both times I was able to force it into 4th gear, drive for about 40 minutes making terrible noises. After that, enough teeth sheared off that it was kinda of quiet. I was able to drive it back home both times using only 4th gear.
Both times 33" tires, 4.10 gears, Mopar MPI on a stock 258.
I finally swapped an AX15 in after 3 T5's in 25,000 miles. About 29,000 on the current AX15 with no issues.
T5 is probably OK on a stock Jeep (tire size, HP, GVWR). But, if you stray too far in any one area you are probably on borrowed time.
I have drove T5's long distances at high speeds in several close to stock Jeeps with no issues.
The AX15 in my red Scrambler made sense for how I drive and use it.
My T5 issues were also probably impacted by the case being wore out in the counter shaft bearing bores, and mainshaft bearing bores. This case finally cracked between the front two bearing bores. T5 case is weak and not alot of meat. No way to improve this design limitation.
That being said, I rebuilt the T5 in my green Scrambler and it seems OK for now (31" tires, 3.31 gears, MPI on a 258 with a bore and cam). This T5 case was perfect and tight. The counter shaft had wear at one of the bearing ends so I installed a used S10 counter shaft (matching tooth counts, better then made in China replacement parts).
My stock 1985 Spring Special CJ7 has a T5 and it is fine.
Depends on how you use/abuse/modify the Jeep. And, if you like pulling/rebuilding T5's once or twice a year!!!!