What rpm at 65mph ???

I could get 54 mph at 2100 with 5.23 rears in my uncles dm600 Mack with a 237, after a little while he let me hop into the kw with the big power 290, that one I could get 57 At 2100 out of with the 13 speed, ran my fair share of 350s and we still have a big cam 400 that runs a bit, with 3.90/13 I can atleast get the modern 65 mph speed limit at 1750ish, mostly everything now is 435-475 with 4.33 or 4.56 rears with the exception of my current truck with a 525 and 3.55s.

 

that’s what I like about taller gears with a rtlo trans, you can leave the big hole for empty/high rolling and cruise in 8th direct at your power band, the one with 4.33 and a 15 over gets to 68 at she’s almost done at 1800, but that’s also only a .79 od, the smaller motors just need all the help they can get

 

I'm not quite old enough to have experienced it first hand, but I remember the stories of guys running out of power on the bigger hills in Pa. Going up rt 8 toward oil city is one I heard about for sure. Kinda ironic, now we've (mostly) got plenty of power, and everyone rides around afraid to load the wagon.

 

As a driver of a t880 daycab 18 speed paccar engine and 3.91 gears, I will add my 2 cents. I'm pulling tankers on overweight permits. I commonly scale 104-105k at the plant when I make milk deliveries. This truck is a 2017 I believe 2nd generation and rated at 500 hp.

With a manual transmission this truck gets done what I need it to get done. It hates steep hills which I drive on steep Wyoming county hills all day everyday. Fuel mileage, you can forget it. This engine is no magic pill that's suddenly going to give you better fuel mileage. With the weight I'm pulling around, the engine is always overworked I get 4.5-5 mpg on a good day. Of course you can't compare my job to many other trucking jobs. I might drive steady speed on an open highway for 20 minutes but then I'm in small towns or on steep hills. I do a ton of stop and go.

A 15 liter would get about the same mileage on my route and give much better performance on hills. If I were making the purchase decision for my route it would be 18 manual with a Detroit DD15 or DD16, or a Cummings X15.... I would look at Detroit before Cummings, I have heard Cummings has reliability issues. I would look at western star, their traditional looking truck I forget what model.

If you pair a mx13 with an autoshift the performance gets much worse. They cant get out of their own way sometimes