rear axle gear ratio

Cummins12V98 wrote:
"Edit: @2100 rpm the new Cummins is likely able to produce close to 360 Hp so it would still be pulling very well down to 2100 rpm/53 mph in 4th gear."

My 33K combined load seems to end up there on a lot of 6 percent grades.

Something doesn't add up. A 33k truck and trailer sitting on a 6% grade in park will have a torque of exactly 2635 lbft on the axle to hold the truck from rolling down the hill. Now if your engine is going to maintain that torque and turn the axle the 472 rpm required to move you're truck 45 mph it will need to deliver 472 x 2635 x pi/16500 or 237 Hp to the rear axle to overcome the force produced by you truck and trailer on a 6% grade.


In order for your truck to be moving at a constant 45 mph down the highway it also requires a certain amount of torque and rpm (power). Assuming your truck gets 10 mpg, it is burning 1/10 of a gallon per mile or 4.5 gallons per hour at 45 mph. 1 gallon of diesel contains about 55 hp hours. Your engine is not running real efficient cruising down the highway so say it is turning 33% of that diesel fuel to rear axle HP. So .33 x 4.5 x 55 = 82 Hp. So your engine needs to provide 82 rear wheel HP to overcome rolling resistance and drag at 45 mph.

So 237 + 82 = 319 rear axle Hp are required to move a 33k truck and trailer up a 6% grade at 45 mph. In all this calculating the assumptions made are the 33% efficiency of your engine and the 10 mpg fuel economy. Both these assumptions are going to be reasonably close.

Now, it seems as though rear wheel Hp is often about 78% of crankshaft power so 319 rear wheel Hp would be equivalent to 410 crankshaft HP. I going to guess your truck would be able to hold 42 mph pulling a 6% grade.