What does rear ratio in a semi means?

Race to the bottom

 

Don’t worry rpm / rear gear ratio is gonna be obsolete…….

 

The sweet spot is where torque and HP over lap. On mine it’s 1600. If I get it up to 1700-1800 before making a run at the hill momentum does help carry it up the hill. But eventually I’ll have to settle in at around 15-1600. Beyond that I’m just making noise below that it a slug. Yesterday I climbed a 7% 8 mile grade at 21 mph in 10th rear on the high side grossing 138,000. My rears are 3.90/5.30

 

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but a 450/12 speed/2.xx will get up the hill faster and get 8 mpg……or so they say

 

On my 510/1850 the torque basically nosedive above 1500, the curves overlap around 1400 or so, and if you have a manual or hold one in gear once you get much above 1500 all it’s doing is making noise and burning fuel.

With my old truck if I did downshift early and ran it at 1600 or so once it dropped down to 1400-1450 the pitch of the engine changed and it would start to pull. If I let it lug down to 1100 or a little lower most of the time I’d only end up splitting out of 18th versus dropping to 16th, and when I was running the same roads all the time with my hopper I began to notice I’d pick speed up quicker after topping the hill in 17th and letting it pull itself back up to 1400 before I split into 18th. Of course that was only at 90k gross so no weight to speak of. And now my experience is based off being 76k or so gross at the maximum.

I put my new truck on the road last January so at this point I’ve had more winter blend and #1 fuel in it plus idling when it’s cold than I’ve had time driving it in warm weather so I’m happy with the lifetime mpg being over 7 on paper right now. Plus I give up fuel economy because I went with a longer wheelbase because I didn’t want it to look real short. So I’m pretty happy with my 510/1850 and 2.79 rears.

 

Years ago I had an old Walmart truck. 430 Detroit and 10 speed direct with 2.89’s I think they were 2.89. Either way it was a pretty decent setup. It was efficient and pulled alright for what it was.

 

Mine is the same power and torque. I beleive the rears are low 3's. I average 6.8 and the motor isn't in it's best shape. I pull 96k one way. Empty going back.

There's one uphill climb. If I'm at 1400 I have to drop a gear. 1700 and it keeps the same gear.

It's not a big climb either.

 

I think if you take care of them they are fine to up around 800,000 to 1Mil without that much trouble. But they need to be spec'd right for it to be ok. If out of spec it will mess up pretty easy at or before 500K.

 

Volvo and Mack have very beefy bottom ends.