N14 or C15 or ??? for good power and MPG?

2.9 rear gears arn't the best for concurring steep grades up or down. Very Slow hard working rough on the engine climb up and makes for a too fast hard on the brakes ride down. Even with a tune i don’t think that spec is dialed in for mountains. Flat land it is. May be cheaper to change rear gears up a quarter. Maybe something in the 3.08 to 3.25 range.

 

Went down Mount Eagle several times, hauling these to Flordy. Drove some of my wife’s family’s trucks, instead of my own. Big mistake. The newer Macks suck for lugging power and transmission options. Oh, and no jakes on these either. Never been much of a fan of Mack engines, much smaller displacement than a cat or Cummins. Seems like they always had to be gear bound, to keep their power. I do like the looks of old Superliners and longer hood R’s.

 

I know you mentioned the oil in the other thread possibly causing your tick. What RPM are you driving this engine at? I mean, going down the road flat @ 70. Are you running in high gear?

Your truck was surely specd for a direct drive. Somebody dropped in a 13od

 

2 things

1. You're driving that truck all wrong. Keep those RPM's up. You need to really keep it in that 16-1700 range. Drop a gear and roll.

2. You admittedly don't have much experience. So maybe you haven't seen hot oil too much. Any decent quality oil is not gonna be a problem. Your engine may use more oil with one versus another, but they are not causing engine noise.

Possibly your oil is being contaminated with fuel. Does your oil level rise?

 

Put a drop of oil onto a paper towel. You’ll see the fuel separate right away. It’ll fan out around the oil.

 

If it isn’t broke, and still reliable, save your money. Get a set of jakes installed and roll. Me and many other folks have rolled to the west coast with a Series 60 and a direct 10.

 

Got yourself a triple digit hammer of truck now!! Lol



Seems to me the mechanic put in the wrong 13sp for the rears you got.?

Can only guess from the speeds/rpm you mentioned you have something like a 16913A transmission?
11th being 1:1
12th .86:1
13th .73:1
That's too deep of an overdrive for 2.93 rears, & the truck probably has start ability issues in 1st, possibly making you use the low hole to lift off.?







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You'd be better off with an RTO 15613. That's basically a direct drive 9sp with a overdrive back box. You end up with 13th being .87:1 for your final ratio. Gives you perfect rpm for a Detroit running 65-70mph in 12th, 1:1. And good rpm for 70mph + in 13th, .87:1..