Model number for 18 speed single overdrive trasmission

Some fleets use those 2.79 gears with 255/22.5 lo-pro tires or even 19.5's....

 

I have a NXS 625 with a BD tune. I changed the rear gears from 3:58 to 4:10 ratio and left the BD tune the same. Truck pulls way better and way easier on clutch and driveline. Fuel mileage is better and I'm runnin the transmission in 18th instead of 17th. Would never go back to the 3:58's.

 

I always thought that the only single OD 18 speed was a RTO instead of a RTLO. I think they only made them for a few years in the early 90's. RTO 16718?

 

Because the most efficient way for the engine to work and put the most power to the rear wheels is in direct drive. In other words, with an 18 and 2.64 or 2.79 rears, you would do your best hill pulling in 16th, which is direct drive. You lose power to the rears by 12% or more in an overdrive gear. You are just burning up fuel you don't have to. The main reason for not going with a direct drive 10 speed is that with the 18, you have all the splits you need clear down into the basement for pulling just about anything you can hook up to the tractor short of a major heavy haul. When you are light or on flat ground and want to step up the pace, you can bust on up a notch to 17th. It you are trying to prove something, you could step up the pace and get in the top hole. At 1500 rpm in 18th with 2.64's, you would be bumping up against 100 mph. Now, that would be crazy. On top of everything else, running in direct drive is easier on all the components and the tranny runs cooler.

So no, it is not crazy. It is well thought out and reasoned for using something like an 18 with such a tall ratio reared gearing.

And I am putting the finishing touches on a glider truck order. It is spec'd with an 18 and 2.79 rears. Only need to make a change to a couple of minor things and the order gets placed, probably by next week. Best of all, that tranny and rearend will be powered by an engine with no EGR, no SCR, and no DPF.

 

That would be assuming running in overdrive. Kinda defeats the purpose.

 

Here are my thoughts on that. You have a truck with a overdrive trans and rears that put the engine at 1500 at 75mph. Now take that same truck change the rear ratio so it will turn the same 1500 at 75mph but do it in direct drive. I doubt there would be a 2% increase in fuel or power let alone 12%. Not worth the other problems that will arise IMO.
I agree 100% with you on your thoughts of buying a glider though.

 

Well those thoughts would be normal, but numbers don't lie. Direct drive gearing almost always beats out mpg compared to overdrive. That is why Wally World, Mesilla Valley Transport, and other fleets are doing it. But they are using 10's. I don't do the simple stuff they do, so I wanted the 18 and will run it in direct.

But your comments on running at 75 makes sense. There is no truck that will get stellar mpg numbers at that speed. But if you put them side by side, going up the same hill, with the same load and everything else being equal, the direct drive version will pull away from the overdrive version. And going down the road, it would take less power to turn the direct than the overdrive. That mean fuel savings. Physics is a hard thing to argue with.

Have you ever noticed, that when a truck is on the dyno, they will test it in the direct drive gear on the transmission? Like 9 in a 10OD, 11 in a 13, and 16 in an 18. That is because it is more representative of the true power of the engine. An overdrive bleeds off some of the power and give inaccurate numbers. That should tell you something.

 

Well, you may doubt there is a 2% loss of power and efficiency by running in overdrive, but the people who make the stuff sure say so. Eaton has put out officially that running in direct can be anywhere from 2% to 4% more efficient than running in overdrive. And, even with an 18, they are on record as saying that if someone runs it in direct most of the time, that they could even get by without a transmission cooler. Eaton has stated that with overdrive, there is a 12% loss of actual horses to the ground at the wheels. That is just one overdrive, now one has to only imagine with a double over what loss there is. Now if that doesn't equate to inefficiency, then I don't know what does. The place to get road speed is at the rear end ratio, not the transmission. The tranny is just a tool to get you to road speed, but the rear end ratio is what should be spec'd for road speed. And, of course, running a tall ratio is not what you would want for pulling toothpicks out of the forest or yanking 100 tons down the road. But then, those guys are not usually trying to run like they are doing produce from California to New York.

But what other problems would arise? Anyone who knows how to drive and is not just a company driver with an attitude can put even 2200 lb of torque to a driveline rated at 1600 lb and not cause a problem. Professional truck pullers have been known to use transmissions that were only rated at 1200 lb but were putting over 2400 lb of torque to them and didn't lose a tranny. Power is all in how you use it. True, most drivers would have an issue, but then, most drivers don't give a rip about equipment, especially when someone else is paying the bills. A good driver knows his equipment, doesn't try to drive like he is at a NHRA national and race from light to light, or have some ego to bruise if someone else can get out of the hole faster. I rather enjoy watching drivers make fools of themselves because they seem to think they need to go from 0 to 60 in 15 seconds, rapping the ceiling with every shift, feel the need to be doing 75 before they reach the end of the get on ramp at a scale, etc. With those drivers out here, who needs the comedy channel?

 

Do you have a link or know where I could find that info? I would think 12% would be for the entire trans. and not just the overdrive. Meaning in direct it may loss 10% and 12% in OD. A 2% improvement I could see that.

I just bought a new Ford F150 it has the 6 spd auto in it. The last 2 being overdrive. Now the automakers are in a big push to meet fuel economy regs and expectations. It would have been as easy and no more expensive to make a desing with lower gears and faster rears but they didnt. If it was as much as you say I think they would have.

It might work for you but I think the first time you have to back a loaded trailer in a tight spot thats up hill or in gravel well.... It may make you look like a rookie with the truck a jumpin
I have 3:55s with a 18 and there are times I wish it had a slower reverse. But that is me I hope it works well for you. If your after every tenth then its probably worth putting up with the reverse issue. I've never been 1 to stretch my mileage to the max. I have a heavy foot. Most of my loads are time sensitive and dont go very far. But then again I doubt I spend half of what you do on fuel in a year.

 

The 12% loss is just in direct. There is upwards of a 30% loss in double over. And auto makers have multiple reasons for deciding on what they will let you buy. Warranties come first to mind, with dolts for drivers as the second. They have to cater to the lowest common denominator and keep their risk down. Also on backing up the truck with a heavy load and tall rear end ratio: How do those with 2.64's and 10 speed direct trannies do it? Especially since the 10 direct does not have the reduction in reverse or forward that an 18 does? Yet, there are thousands of trucks each day with 2.64, 2.79, and 2.85 rear end ratios that back thru all kinds of stuff, and don't look like rookies.

a spec guide from eaton (see direct vs OD section):

http://www.roadranger.com/ecm/group...roadranger/documents/content/rr_rrsl-0001.pdf

A quote from the senior engineer at Eaton during MATS 2011 (mid article):

http://www.bigrigowner.com/high-performance-diesels-reminiscing/

"He did go to the Eaton Driveline Booth and spoke with the head engineer and was told that he would gain at least 3% more fuel mileage with the 2:64 gears and running in direct. He was also told that he would NOT need a transmission cooler if he drove in direct gear. You see, like we having been saying for years, direct gear, less heat, more horsepower to the rear wheels and better fuel mileage and much better pulling power in the mountains. It's just the opposite of what most of you think."

Bottom half of this article:

http://www.bigrigowner.com/egr-breakthrough/

"Many owner-operators think their 13 and 18-speed Eaton Transmission is a single overdrive and when they split down ½ gear they are in direct. This is not true; you must go down 2 gears or 1 full gear to get into direct, which is the work gear (the gear you should be using to pull a hill). All 13 and 18-speed Eaton transmissions today are double overdrive when there is a 3:36 to a 3:73 gear ratio rear end. When you're pulling in direct gear, only about 11% to 12% of the power is lost in the drivetrain and that is why when a truck is on a dyno it's always run in direct gear. So let's do some calculations and see how much power is lost in 13th. or 18th gear, vs. direct gear. A 550 Caterpillar with 3:55 gears puts out about 490 hp to the rear wheels in direct gear and that is a 12% loss of power. Now if 30% of the power is lost in 18th gear, then only 390 hp would be delivered to the rear wheels. That is approximately a difference of 100 hp when pulling in double overdrive. The fuel mileage will improve by at least ½ mpg running in direct gear with 2:64 rear gears."

DL550CAT, don't try and assume that I have no experience with any of this by comments like
"I think the first time you have to back a loaded trailer in a tight spot thats up hill or in gravel well.... It may make you look like a rookie". I have been at this for 3 decades, one of those which was spent in Alaska year round, before Ice Road Truckers show was glint in anyone's eye. I was driving 18's in the 80's in the Arctic. The 18 is able to not only have a high and low in reverse, but you can also split it. The reduction in low split reverse is more than any 10 or 13 would have. Makes up for the taller rear ends. And the deep reduction of the 18 on the forward side makes up for the taller rears as well. True, you would have to do most of your starts in a lower gear. And this setup in my truck is not trying to pull frac sand in the oilfields or haul logs out of the forest. It is doing general freight, dry box stuff. I made it clear in previous posts that this would not work for every application. But it IS more efficient than any overdrive setup for the majority of what most people do with trucks that get on this forum. And there is a lot of owners that have spec'd 18's with taller ratios and are doing great with them. Some of which are pulling heavy on a regular basis and tanking fuel and having to back into some interesting areas.

It is just that sooo many have been conditioned to think a certain way, it is difficult for them to accept any other method.