Rear ends and transmission compatabilities

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.

 

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....

 

Cowpie1 i live in Washington state i haul a lot on i-5 to cali and to Dallas Texas. I have 2006 565 cummins with 18 speed 3:36 I want to swap my rear ends for better fuel mileage what do you think the best rear end ratio i should use Thanks.

 

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.

 

How would it leave me only a couple of gears to split by running in 16th? I have splits all the way into the basement and a couple of gears on top for faster running. And on harder pulls on the hills, I never get out of top range anyway, so it is non issue as well. It is a common misconception that you have to use each and every gear on top. Many 18's and 13's in the past were only single overs, which would equate to 17th and 12th respectively. While I may not have as many to split on top, doesn't mean anything. Even with this setup, I only split the bottom occasionally like most folks. Most of my bottom shifts are full shifts, with only splits when I get in top range. On more challenging loads, I may split a couple of them on the bottom. While I may have a couple of gears fewer on the top under normal operating, I also have the advantage of extra gears on top when I feel the need to pick up the pace. As for Detroits getting their best mph at 1600, would have to dispute that a little. Mine does exceptionally well at 1400 rpm. Mid 7 averages with summer averages in high 7's and low 8 mpg territory. Not something to sneeze at. Even fighting this latest midwest winter storm, I pulled off darn close to 7 mpg for the week. There is definately an advantage to running primarily in direct drive (16th), at least in my situation.

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.

 

Thanks for answering my question im a new o/o and i will be posting some more questions in the future thanks for the help

 

If you go back to the OP post he has a direct 10 speed.........The OP wanted better power with MPG...now putting in any OD is just throwing money out for an O/o look alike...Now one thing i didnt think of was he could bolt in an old style 13 direct or maybe a single OD 13 but with those gears he still may have the same issue with the single OD. The great thing about a 13 or 18 is the split thats what makes them nice but on the big hills if you are already down a gear or 2 and you have that long pull your down on the low side burning fuel and slow as hell....You need to spec your gears to your motor...you want to keep in your power range as best a possible....ie my 2000 classic was getting 5.5 to 6 mpg with a super 10 3.55 and low pro 24.5 now i changed the tires to low pro 22.5 which gives me the same effect as if i had put 3.73 in with the 24.5 now my rpms came up i was 1400 at 65 now i am 1600 at 65. i also went up to 6.6 to 7.4 mpg this is an 12.7 hood and we normal run 70 or better.....and getting good mpg still....now if i drop a 13 behind that and put 3.73 with the 22.5 i might get alittle better...but on my other truck it had and has again a 10 direct. while i rebuilt it i put a double od 13 in it. it had no power and MPG dropped to the 4s.....that truck has 3.08 rears direct 10 back in now and 12.7 to low of an rpm and your motor works to hard. to much shifting its working harder. all adds to more fuel burned. just like if you idle your truck. fill it up idle and normal idle fill it up. than idle it the same amount of time but bump the idle up to 8 or 9 and it will use less fuel....its all about getting EVERYTHING to work TOGETHER. thats why alot of guys balance the whole motor out match their trannys to their rears or vis versa....you say the motor does not know whats going on behind it....BUT it really does. the ecm puts everything together along with your input to tell the motor what to do....when your pushing that pedal and shifting alot its telling the motor to work harder not easier and that inturn burns more fuel....sure you could run it in direct gear to keep some power but why not do the job the right way and get everything to work together. a build with that setup is pure speed not power....would be great if you wanted to run 150mph

 

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?

 

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