18 spd vs 15spd/9LL

Need the whole trans model #'s to make that comparison.

 

18 speed. Only way to go. More versatile. I believe the typical 18 has a lower starting gear than the 15's. The 18 has 18 useable ratios vs 12 for the 15 speed. 15 can't be split so its basically a 10 speed when being driven on the highway. A modern 18 speed can handle like 2,050 ft-lbs. I think most 15's are rated less than that. I know my old 15615 is only good for like 1,500 or 1,550.

 

The first two numbers are how many hundred foot pounds of torque the transmission is rated for. If you reduce that your backing up.
If a lesser rated transmission is better rated for you application I can not answer. I know that sounds stupid. As in the case of a drilling truck with a splitter box, kind of like a bownie, with two output shafts. One for the truck and one for the drill. It normally ran in third gear drilling. When it was uprated to a different model trans it would not stand the high sustained torque in third gear.

 

RTLO18918 Will handle all a 550 cat can give it.
If that's what you were running

 

Ratings aside, I've seen some 15 speeds that live a long long time behind some hot hot 3408 cats and 600 KTAs.
There a very strong trans that take a lot of abuse.
Having said that a good rtlo 18 would be every bit capable.

Yours was prob just that, wore out.

 

The 18918b will handle anything you can throw at it as previously mentioned. I just put on a stage1 turbo and a Mr. Haney tune in my C-15 and soon will be putting this trans in my truck. My truck is putting down a lot more power than a stock 550hp 2050ft/lb torque motor and I've talked to many knowledgeable people and this is the trans along with a 2050 eaton clutch they suggest