18 speed vs 15 speed

Go to Eaton's website and look up the gear ratios. I had a 15 for several years, and the only problem was it is kinda gearbound. The 18 has a higher top 2 gears, and you won't have to go real slow to drop into the low end. The deep reduction on a 15 is quite handy for starting on hills with a heavy load, also for backing.

 

13 speeds and 18 speeds have come a long way since the early 90's.
Back in the day, 15's were quieter, less temperamental, more reliable. Gear selection was 2nd to a 13 or 18.
Now...18's and 13's have evolved in the quiet/reliable category. ...such that the 15's are obsolete because of gear spacing.
If it's an RTLO 13 or 18 (not RTO) you have yourself a good trans.....

 

Wow, lots of opinions. Nice to see everyone posting nicely. I have trucks with 15 and 18 fullers. Here is my take, as some have said in above posts. I prefer the 15 for when I have to go off road, and the 18 for highway. I really like the 300 rpm splits myself for heavy haul. If I didn't have to pull the #### ting back over and down to get it into 6th, I would say it is hands down better than 18, but that is just me. The 300 RPM splits are great for rolling down the highway heavy. Remember your top two overdrive gears will cost you about 25% torque.

If you have not driven a 15 speed, you need to remember when you come out of deep reduction to low range you go to the next gear down in low range. Both transmissions are very good, just depends what you want.

 

I got a 15 od in my w9 and I'd rather have an 18 to be honest.

 

Can't split gears in high range with a 15 that's why they are obsolete.

 

That's part of the reason I prefer the 18. On the 15 the ratios seem super tight in the low range and once you hit the high side they seem to spread out.