Onezman said: A 10 speed will get the job done 99% of the time for all OTR applications. 18 and 15 speed transmissions are becomming a thing of the past in the market place.
Generally, 18 speeds are only used for very heavy haul applications. These transmissions were popular when HP ranged from 250 to 350 with torque ranges from 1050 to 1250.
With the advent of higher HP higher torque engines, that many gears are entirely unnecessary.
For RV porposes, even a 10 speed is too much. The lower gears will almost never be used. For towing, 4th gear is sufficient for starting off. I use 5th gear starting off bobtail. The only time an RVer will use lower gears is for "creeping."
**
Actually, there is a different basis. The 18s are pretty new, derived from the 13s in the early 90s, when the 3406B and Series 60 were changing the rules and did a decent job, with adequate torque and wider powerbands.
The 5 & 3/5 & 4 2 box transmissions are more like the situation you are describing - a response of the 60s to the lack of power, and at least as important, the narrow power bands, in the OTR trucks of the time. In the 70s, the folks who had the K series Cummins had to have such trannies because that was the only way to handle the torque of those large and heavy motors. Now Fuller and others make trannies that can handle a lot more, but a friend of mine saw a Fuller 10 blown up during a dyno testing and tuning session of a 6 cylinder Mack in the 70s.
For many purposes, a 10 is now adequate. However, for people hauling max loads (80K, not heavy haulers), the ability to split gears is of great value. The gap in a 10 speed is pretty big, and can make a hell of a mess on a grade. I ran out of the midwest, and a 10 is adequate there, with enough power. I would not have one for most purposes if I ran as far east as PA or West as Colorado; the ability to split the top gear on a pull is great. Depending on how much improvement there has been lately, I might reconsider that. A 10 was adequate for me running a V8 Mack in the late 80s, but it had far more torque than anything else around. I could run up grades at 65 that others could barely top at 30. (I had to jake going UP Jacob's ladder on the Mass. pike to keep down to the limit, and could get away with using the car lane on Sideling Hill in PA, running at 60+ when almost everyone else was at 20-25.) I see trucks having a hell of a time going up Vantage to Ryegrass (I90 W/B, from the Grant-Kittitas County line in Washington); often they are down to 40, which is inexecusable. I saw a heavy hauler from the Midwest, obviously underpowered, down to about 10 MPH.
Downshifting sucks fuel, and my time with a 9 speed and a 350 Mack (able to pull like a 400 Cummapart of the times) was not fun in some ways. The way to make speed and have decent fuel economy in a truck is to get a decent speed, like 62, set the cruise, and never slow down or downshift.
15s are not at all like 18s. 18s are good for heavy haulers due to the small spread on each shift, but it takes a lot of weight to make that important, and that's not our situation here. 18s, like 13s, are based on the 9 speed. In the 13, one simply can't split the bottom 5; in the 18, each gear can be split. A 15 is actually a 10, with a lower range below it, allowing for more multiplication at low speeds. (Besides, the max rating for a 15 is 1650 ft-lbs, which is a small motor in the OTR setting unless one never leaves the midwest.)
For an RV, it is almost certain that a 9 or 10 speed is ample, with one exception. The rule for picking a starting gear is that you use one that allows you to let out the clutch at idle (NO THROTTLE) - you have to get the clutch out and keep your foot off it, in fact have it flat on the floor next to the pedals! This should not be a problem with the relatively light weights involved. However, a nice low gear option (forward and reverse) would be great in a tight spot such as an RV park. The Mack trannies have it all over all the others in this issue; they have options for super low first and reverse gears. The Fuller 15 would be good on this issue, for that reason. Other than moving around in a tight spot, one need not use that creeper range, so it is really a 10 speed. Less shifting is a good thing for RV use, as it can be hard on the clutch if one is enough of a fumble bum to need it (although shifting is a LOT easier when one is closer to 80K than when light). This would be a good argument for an autoshift.
Today's powerplants have a LOT more torque, get better mileage, and broader powerbands. The first truck I really drove OTR, when a driver got sick, was typical of the old style. It was a '77 Freightliner, with a 400 Magnum small cam II and a 13 speed. The damned thing had no powerband. It ran like a scalded cat if you kept in the power band, and with 4.33 gears would easily pull 80+. However, it had to be kept perking like a 2 stroke racing motorcycle. The powerband was 1800-2100+, and at 1798, it fell on its face. Those days are over. The last of the N series Cummins (the same block as this motor) had a lot more power, a power band well over twice as wide, and was more tolerant of the drop in RPM.
What we have here is a guy looking at a used truck for his starter. It won't be perfect, but he (like me) does not know enough for sure to go order one and get specs right (I'm still not sure what I would spec for an engine/tranny combination; it depends on the feasibility of a driving front axle and who will make a truck with one). An 18 auto will skip gears as needed; it has a clutch (critical - a full automatic is a BAD idea because of the torque and heat involved, although a much smaller risk in an RV); and it is already there. For my money, the idea that an 18 would be silly to order in a new truck used for an RV, which is clearly correct, is not relevant here.
