I see you have limited experience with what we are talking about here, using tall rear gears and comparing 10's and 18's. An 18, running in direct (16th) has more functionality than a 10 direct ever could. That 18 (not counting the overdrives) gives you the flexibility of 16 gears to use depending on what you are doing. The very bottom reduction is almost identical to the bottom of a 10 direct. Plus, if one steps up the pace (like we all do once in a while like empty or bobtailing) that 18 allows one to go on up to 17th and strut at a decent clip while keeping those RPM's nice and low. And the 18 will handle far and away more torque than any current direct 10 being made.
So, how much good one sees from this kind of setup, depends on what one is doing with it. For most normal folks running only 4 lanes with average loads, it might not be worth it. For those that regularly pull 80K gross, and do it on a lot of two lane, winding, hilly roads, it can make a world of difference. Especially when you are putting down over 1800 lb of torque and above 500 hp. No direct 10 offers torque ratings that high. It is a lot easier on equipment to keep things in a nice tight 250 RPM band with an 18 and tall rears than it is to do 400 RPM splits with a 10.
And so what if the overdrives are hardly ever used? That saves quite a bit in parasitic power loss associated with those OD's and keeps more of the power getting to the hind end. I may use a few more gears lower than the OD's, but the reverse is true..... with shorter rear ratios, one is not using some of the lower gears all the time. I guess one could look at an 18 as a waste with tall gears, and I would to, if I could get a current manufactured 16 speed direct or a good 10 direct that would handle higher torque and hp ratings. Since I cannot get either of these, that leaves an 18 or, at the very least, a 13 under the right conditions.
And the 18 I put in this truck didn't cause me to mortgage the home. Sure it was more than a direct 10 would be, but for that extra dough, I get a lot more flexibility and usefulness. Kinda like buying a regular flip phone or an iPhone 5. You will pay more for the iPhone, but it will do a lot more than the flip phone ever could. But they will both make calls equally. It is all about what you need.
Rear ends and transmission compatabilities
Discussion in 'Freightliner Forum' started by Mykids32, Oct 18, 2012.
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Rideandrepair, hamboner, Autocar and 1 other person Thank this.
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cow i am not saying that an 18 or 13 is not a better tranny. as for how the 10 will handle my direct 10 is bolted behind a 500 detroit push 575 hp on the dyno. so far so good. what i am saying is what you said in your last post. the 18 and 13 top gears dont really mate to the rears. now as for running lower rpms our 2000 freightliner with a super 10 and 3:55 runs down the road 70 mph at 1400 rpms and gets crap for fuel. my buddies w900 with 3:36 rears and an 18 runs down the road at 70 at 1200 rpms and gets crap for fuel. only setup we have seen get fuel mpg at lower rpms are the new cummins in which we have not seen hold up very well pulling heavy. cats and detroits tend to gget their best around 1600. my whole point is if someone like me who has a factory direct tranny wants a 13 or 18 they need to plan on regearing the rears to get them matched to the tranny. you are 100 percent right about the tranny and pulling. being able to split the gears helps alot but if you dont have the right gearing you wont be in a postion to split the gears. your running 16th which is direct that only leaves you a couple of gears to split. now if you re geared and got it where you are running 18th with the same rpms as you are in 16th than you have your intire top side to split on those long hard pulls which is where the 13 and 18 show their greatness....
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Well, it all depends on how fast you want to run and size of tires. Given the I-5 corridor, you are going to be limited on the top end. Ever try just running in 17th at a moderate pace and see what the results are? Would have those rpm's at a better balance for power/economy with that ISX. On low pro 22.5's, in 17th with 3.36 rears you would be at 1400 at 56 mph. On tall 22.5's you would be at 1350 at 57 mph, probably a better solution with that one. 1400 you would be at 59-60 mph. Not a bad combination for pulling some grades and still keeping from rapping out the engine. And a low cost alternative to changing out rears. From my experience with the ISX, it got it's best mpg in the 1350-1400 rpm range. You would still have a gear up if you want for faster running.
If you are wanting to stretch it out on the speeds for Texas and such, then a rear change is definately in order. 3.08 or 3.25 would be pretty decent choices. 3.08 in the top hole on low pro rubber would give you 70 mph at 1350. Of course, if you are going to run those speeds, better mpg is going to be a problem no matter what you do, unless you run lighter loads. -
But each case is different. And what I haul and how I do it is not the same as someone else. But the fact remains, running in overdrives actually takes power away from the rears. The parasitic loss of power can be substantial in double over. Why do you think that when on a dyno, the most accurate test of how much power is being put to the ground is when the tranny is in direct? That should cause anyone to wonder why they are not using direct more often. And it must be more efficient, as even Eaton will confirm that a transmission runs cooler when you keep it out of the overdrives. If you want to set land speed records, then overdrives will be mandatory even with tall rears. But that is defeating any purpose to maximizing efficiency.hamboner Thanks this. -
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Hello all. This is my first post on the forum which is mainly a question.
I'm buying a nicely loaded up (800,000 mile 2007 T-600) (200,000 mile recon 475hp 1850lb ISX) (13-speed 1850lb .73) (3.36 rears) (lp-24.5 tires) (240" wb 86" studio). Does anybody here know if this setup would work well for running the mid-western and eastern states pulling a fully loaded (80,000) flatbed. There would be times of being fairly light all depending on my freight situation (maybe 25%-50% of the time including deadhead)?
It would be nice to get Cowpie to chime in or somebody that knows from experience. I'm thinking the gearing could be better with lp-22.5 tires. Or, maybe 11r-24.5 tall tires and running in 12th (.86) would work well (then I may lose fuel mileage from increased rolling resistance). I really wouldn't want to change the rearend ratio until it was time due to wear (I realize 3.55's may be better for my freight situation/region). I haven't driven the truck yet; however, that may be the best way to find out. The previous owner who originally spec'ed out the truck pulls van freight on mid-western to western long haul routes (I don't know what weight they were hauling).
Does anybody know why Cowpie got out of his ISX and changed to a long legged Detroit?
What I've become used to and what has worked well for me is my (1,100,000 mile 2000 127" 379 Pete) (never majored 3406e 550hp) (18-speed .73) (3.55 rears) (11r-22.5 tires) (275" wb 63" flat top) which is now parked in the driveway with coolant leaking into the oil pan (liner o-rings may have failed). It still runs like a top and it gets great fuel mileage - always 6+mpg yearly average including idling). Instead of choosing to major the engine at this time (in order to replace liner seals and most likely damaged bearings), I've decided to finally take the plunge into owning a newer egr engine truck (no dpf).
I may have to post this in a new thread; however, I'll try here first.
Thanks for any input or advice. -
I have a 12.7L Detroit with 13 speed tranny and 3.55 rear end gears and low pro 22.5. Final drive ratio on tranny is .87 and 12th gear is 1 to 1. I run I-20 from Dallas, Tx to Louisiana speed limit is 75mph. At 65mph I’m running 1700rpm. 70 mph is 1800rpm. If I changed my rear end ratio to 2.93 would that lower my rpm? Will I still have enough pull power to pull a few hills?
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Final on a 13 spd should be .73. With low pro rubber you should be at 1500 at 65 mph. Yours sounds like 3.79
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