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Heavy Duty Truck Gearing for High torque Diesel Engines

If your truck is geared wrong for your job, you may not have much of a truck.   Fuel is wasted for the truck geared too low, maintenance and repairs are in the future for the truck geared too high. 

The truck that never gets off hard surface or almost never starts on a hill works for the faster ratio. This truck geared too low could  become "gear bound' (engine and gears wont spin any faster) at top engine RPM and might result in not being able to get up to the desired driving speed.

 More common; a truck geared too fast and expected to operate heavy over soft lots, roads or steep grades.

The industry term is "startability"; getting the load moving from standstill for a given grade and surface type.

.  If startability is wrong for the job, the problem is a big one.   Changing the trucks transmission, rear axle ratio or both is the solution to get the truck in balance with the job at hand.

The key most often with startability issues is: what is happening with the clutch?  Clutches are expensive to replace and once replaced, the initial problem that caused the failure is typically not resolved.  Thus, another clutch replacement may not be far into the future.

A simple Rule of Thumb guide that has worked for me 20 years;  multiply the low gear of the transmission by the rear axle ratio. If your on soft soil, at all, this result should be greater than 40.  Most dump trucks for example will be in the 45-50 + area.  A truck with 36 or so ...offers almost no use flexibility at all.  ...don't park, loaded on a grade, or a soft spot.  You might not get started. *

Bigger rear axle numbers mean better ability to "pull", or get up a hill.  Smaller rear axle ratio numbers mean better ability to get the best fuel mileage.

Nation wide, the 3.55 ratio is the most popular ratio for most road tractors (gear fast run slow)  Generally 3.36 is considered too fast, but may become more popular with the new Cat ACERT engines. (ACERT gets torque and best fuel mileage at very low engine RPM's)  The transmission high gear then;  is normally a .73 or .74 overdrive. The 3.55 ratio is not generally considered a good ratio for dump tractors for example. 3.70's might be the better choice.  For a truck (hauls most of the load on its' back) 

Ratios like 3.70 and higher are then considered pull side ratios.  They expect slightly less overall fuel mileage for each notch above 3.55 and a corresponding improvement in hill climbing ability.

The upshot of this discussion is consider the surfaces you will encounter and the load carried, balanced with the trucks "gearing",  if you want the most reliable truck to fit your job.  

truckDoctor

*the trend has been higher and higher Horse Power  engines, with a corresponding increase in torque. In theory these increases can overcome the load on a given traction and grade for a given ratio.  The problem becomes all the stress in-between and things can and too often do break.  

http://www.roadranger.com/products/trans/trans.htm is the Eaton Fuller page to get to current production transmission  ratios.  If you have better luck finding transmission ratios on the web, let me know and we will post them. Or maybe, I get ambitious and compile what I have in literature and books.   hmmm.