Semi Truck Transmissions: Types, Gears and Brands Explained
A semi truck transmission is a heavy duty gearbox built to carry 1,450 to 2,250 lb-ft of diesel torque to the drive axles. It comes in three kinds: a manual with 9 to 18 speeds, an automated manual that shifts the same kind of gearbox by computer, and a fully automatic with a torque converter. This guide covers the types, the gear counts, the brands, and how to find the right one for your truck.
Types of semi truck transmissions
Manual. The driver shifts with a clutch pedal and a lever. Heavy duty manuals are twin countershaft designs, meaning two countershafts share the load, which lets a compact case handle high torque. The Eaton Fuller Roadranger line is the most common manual in North American trucks, alongside Mack transmissions in Mack trucks and Spicer TTC transmissions in medium duty.
Automated manual (AMT). A manual gearbox with a computer, actuators and an automated clutch doing the shifting. There is no clutch pedal for normal driving. Most new Class 8 trucks now ship with one: the Eaton Cummins Endurant, Volvo I-Shift, Detroit DT12 and Mack mDRIVE are all 12 speed AMTs, and Eaton also builds automated 10 speed, 13 and 18 speed units.
Automatic. A planetary gearbox with a torque converter, like the Allison 3000 and 4000 series. It shifts under full power without a break in torque, which is why refuse, fire, mixer and other stop-and-go vocational trucks use it.
How many gears does a semi truck have?
Most semi trucks have 10 forward gears. A heavy duty manual gets its gear count by stacking sections: a five speed main box, a range section that flips it from low to high, and on 13 and 18 speeds a splitter that divides each gear into two half steps.
- 9 speed: eight road gears plus a deep low, popular in vocational and regional trucks.
- 10 speed: the standard over-the-road semi truck transmission, direct drive or overdrive.
- 13 speed: split high range for heavier loads and hills, a favorite with owner-operators.
- 15 speed: a 10 speed with an extra deep reduction for starting heavy loads.
- 18 speed: split in both ranges, the choice for heavy haul, logging and oilfield work.
Fewer, wider gears are simpler to drive. More gears keep the engine in its best RPM band under a heavy load or on a grade. Medium duty trucks run 5 and 6 speed transmissions, and dump and construction trucks often use an 8 speed with two low gears.
How to read an Eaton Fuller model number
Take the RTLO-18918B. R is Roadranger, T is twin countershaft, L is low inertia and O is overdrive. An added F, as in RTLOF, means a forward shift lever position. In the numbers, the first two digits are the nominal torque rating in hundreds of lb-ft (18 is about 1,800), and the last two are the forward speeds (18). The letter at the end is the design version. The data tag on the passenger side of the case carries this model number with the serial number and TA number; our guide on how to identify an Eaton transmission shows where to find it.
What fluid does a semi truck transmission take?
Eaton Fuller manuals take Eaton PS-386 synthetic transmission lubricant, and using it is part of Eaton's extended warranty programs. An RTLO-18918B holds about 28 pints. Automatic and automated transmissions have their own fluid specifications, so check the model before you fill. Other lubricants are under transmission fluids and lubricants.
Signs your semi truck transmission needs work
- Grinding or hard shifting, often worn synchronizers in the range section, a clutch that is out of adjustment, or a failing air system.
- Jumping out of gear, usually worn sliding clutches, gear teeth or shift forks.
- Whine or growl that changes with road speed, a sign of worn bearings or gears.
- Range or splitter will not shift, most often an air valve, air filter regulator or O-ring problem.
- Leaks at the input, output or cover seals, which starve the gears of oil if left alone.
If the problem is the clutch rather than the gearbox, see how to adjust an Eaton clutch.
Rebuild, remanufactured exchange or new?
Rebuild it yourself when the case and main gears are good and the truck can be down for the repair. A basic or master transmission rebuild kit covers the bearings, seals and gaskets, and individual gears and shafts are under transmission parts. Exploded views for most models are free in our transmission parts manuals.
Buy a remanufactured exchange unit when downtime costs more than the difference. It arrives rebuilt and tested, you swap it in, and you return your old transmission as the core for a core charge refund, subject to the manufacturer's core policy.
Buy new for a new build, a spec change, or when you want a factory-new gearbox with the full manufacturer warranty.