The reason semis have manual transmissions, is that they were more reliable and longer lasting than an automatic, which uses a lot of consumable parts that wear down easily and need to be replaced more often. This is purely a factor of maintenance cost, however, current technology has improved and automatics are now being used more widely for two reasons; the maintenance costs have declined and to encourage newer drivers the ease of learning to drive an 18-wheeler without having to learn the subtleties of shifting and double-clutching through 8-15 gears. The reason a manual on a semi requires double-clutching is also due to maintenance costs. They have no synchronizer gears, which are made from a malleable/soft material that easily wears out. To alleviate this, they are removed, which means the driver must manually synchronize engine and drivetrain speed by clutching once to change gears to nuetral and a second time to change to the next gear.

Working the trucking industry, I've seen alot of both transmissions. There are actually two different types of automatic transmissions for semi trucks. One type is fully automatic just like a car and only has a brake and fuel pedal. The other has a clutch pedal that you use for starting out and stopping but shifts on its own once you get going, called under one name as an Eaton Autoshift. Also if you have ever driven either of the automatics, you will notice that a transmission ECU will adjust the engine RPM to get it to go into gear. Ben91069 touched on it, were a semi transmission does not have a gear sychronizer, so engine speed has to be the same as drivetrain speed to shift smoothly. It is the same basic shifting as in a manual transmission. Some people use the double clutching method, but more and more people inclucing myself, "float the gears" where you shift based on rpm without clutching at all. I have also heard drivers say that manual tranmissions are better for mountain driving and heavy haul but they are both out there in the trucking industry. One is not better than the other and both have a purpose, manual is just more popular.

These days, the answer is that that is how it's always been done. Today, there are very dependable "auto-shifting" transmissions, which are actually the manual transmission with electronic and air-powered shifting. This is becoming more and more common, largely for safety reasons (a driver can concentrate on his turning instead of having to pay attention to gears), but also for fuel economy and lowered maintenance. My truck has a 13-speed manual, but if I were ordering it, I would order one of the auto-shifting transmissions.

Most of the older drivers dont like automatic transmissions because they like to have control of the gears when climbing or going down steep grades. And the older automatics were really not very good. But in the last 8 -10 years they have really made some great imporvements on automatic transmissions. However, when I started driving, 1975, we just learned how to "float the gears", (match motor RPM with transmission RPMs) and change gears with out the clutch once you start rolling. It is not good if you cant do it without grinding the gears. But I have been an owner/operator for many years and have never had a transmission problem from floating gears. I might get an automatic next time, however, they have really come a long way since the old days.

Well I can tell you being a mechanic, if i were to buy a truck i would stay away from the auto shifts, at our shop in the winter months you get green corroded wires, the truck will not shift into high range. Or the electrical motors on the top of the trans stop working, than it don't shift. How ever for a fllet that just wants to get guys out on the road the auto shift is the way to go, less wear on the clutch that you can get from a new guy that can not shift. But for a broker that i would think can shift with out the clutch i would stay with the old manual shift, less parts more money in your pocket

DAA it helps get moving with a very heavy load, and automatic transmissions are very heavy, weight is a factor in most 18 wheelers, the more the truck weighs, the less payload you can haul