T300 Gearing Stinks

[P51bombay]

Having been in the T3 a while now, I gotta say the gearing is shall we say........lacking. Don't know where or what this truck was specked for but it wasn't made for these parts. (got a stretched frame too) Set up with a 3126/250, 10spd OD(.75) 11R-22.5 drives and a 4.63 RE. Its getting about 1 mpg worse than the M2 as well - 8.8 mpg on a good week. Seems to have been geared for 70mph but the speed limits around here are 60/62mph leaving me @ 1740 at cruise and struggling on most anything bigger than a gopher mound - I had the speed limiter bumped way up which has helped - if I get the rpm up to 2000 (70mph) I can get up hills ok, otherwise rpm drops below 1500 and I loose 20 or more mph and/or have to drop to 9th. Talked with the shop and while the labor to change the RE (R&R with a reman) isn't bad at roughly 2hrs, the price of the RE was less than appealing. So plan B might be switching to LP rubber out back. Using the Cummins calculator and plugging in my numbers I get 1793rpm which is close to the 1740 I actually have. Playing with tire revs per mile on in the calculator I come up with the following

540 = 1938
545 = 1956
550 = 1974
555 = 1991
557 = 1999
560 = 2009
565 = 2027
570 = 2045

Based on the old truck, same engine but with a 6spd which was about 2000rpm at 62mph and since the calculator is just slightly high it appears I need a tire with 567 revs per mile to achieve that 2000rpm target, though anywhere between 555 and 570 would probably be fine.

Looks like the choices would be:

255/70-22.5 around 567revs/2038 engine/Michelin XZA
255/80-22.5 around 543revs/1948 engine Michelin XZU4
265/75-22.5 around 540revs/1938 engine/Bridgestone 726
275/70-22.5 around 547revs/1963 engine/Michelin XZA

thoughts/opinions?

 

[spudhead911]

I've always liked the Michelin over the Bridgestone.

gearing between the tranny and rearend is everthing.

Most of the expediter straight trucks are streached unless custom ordered from the factory.

I never liked class 7 trucks for expediting, especially with a team. I owned two class 7's, I finally wised up and bought class 8 trucks. The first one was a left over 2000 Century I bought new in 2002. It had a Cummins 330/370 hp ISM in it with a rockwell 9 speed direct drive with .322 rear twin screw. I got right around 11mpg with it. The truck would go just as fast up a hill as down. When running with an escort would always have to slow down going up hill so I wouldn't leave escort in the dust.

In 2007 we had a Western Star built. it has the mercedes 4000 series engine set at 370hp and 1450 ftlps of tourge. It has an Eaton UltaShift 10 speed tranny with a .342 single screw rearend. We average right around 9.5 mpg. It has the pre 2007 emision engine in it, no DPF. rpm at 65mph is right at 1450 rpm. Great truck. Would never go back to a class 7. The cost of the truck was the same as a class 7 sold by another dealer.

 

[piper1]

Proper gearing for a 3126 will give you 2000 rpm at 60 to 63 MPH. This explains why the engine feels better to you at 70 MPH in your case as that is 2000 rpm.

Changing to a 255 or other small tire may come back to bite you. These tires are not always readily available on the road if needed. Also these smaller tires come with a reduced load rating in most cases.

A 4.88 rear end will give you 65 MPH at 2000 rpm with regular 11R22.5's and 63 MPH with 275/80R22.5's. (std low pro's)

You may want to investigate a rebuilt rear end (dunno if that's what they quoted you) or perhaps find one at a wrecker. 4.88 is a common ratio in the medium duty world so they are out there. The next step up is a 5.25 (I'm assuming your RE is an Eaton). That is a big jump.

The price of 6 new tiny tires (yep you would need to do the steers too) is likely going to be $2000, that might go a long way toward a rear end and still leave you with common tires.

Ideally it would be great to go std low pro 22.5 and a 4.88 but that would be some $$$$.

Just a thought.....

 

[P51bombay]

Hadn't thought of tire availability so will have to check shops in the area to see what they stock (local truck) but using Cummins powerspec calculator shows that a 4.88 RE will only bump rpm up about 100, I would need to go 5.29 to get hit the 2000rpm target. Why would the steers need switching too? I see many of the Interstate tractors that go into Canada run std steers and LP (255's) drives to keep their trailer height down. Prices I got (apart from labour) were pretty rough so maybe I will do some checking and look into a wrecking yard price as well - I'm not thrilled at loosing ground clearance either as with the bags dumped and my rail rate down it would probably hit the ground.

Other than getting up hills I actually DO like crusing at the lower RPM but mileage and drivability says otherwise.

 

[FREE TO FAIL]

whats this truck weigh? Seems to me that the engine is really underpowered for anything appoarching 25k 250hp puts you in the hino performance catigory (ugg) I would dump the truck! I would guess that your never going to get it running well as its working to hard. With a 3126 I am also guessing you are probably near the end of its life cycle anyway (get it dyno'd to see what the rhp is!) these are generally really crappy motors. Buy the same truck with a cummins or detroit with 350/400 hp and you will get the performace and fuel economy your looking for. I am curious whats the torque figure for this motor?

 

[P51bombay]

Not sure what the torque rating is, never seen a chart or the number on the truck. Empty weight is about 18700. Its only got about 191K miles on it so hardly worn out, although the clutch is down to about 30% - other than that its in pretty nice shape, it was an ex Paclease truck and they actually did maintain it fairly well. Being the first truck I've owned its main purpose is to pay for itself and build a nice deposit on a new one - what that will be remains to be seen.

 

[piper1]

Hadn't thought of tire availability so will have to check shops in the area to see what they stock (local truck) but using Cummins powerspec calculator shows that a 4.88 RE will only bump rpm up about 100, I would need to go 5.29 to get hit the 2000rpm target. Why would the steers need switching too? I see many of the Interstate tractors that go into Canada run std steers and LP (255's) drives to keep their trailer height down. Prices I got (apart from labour) were pretty rough so maybe I will do some checking and look into a wrecking yard price as well - I'm not thrilled at loosing ground clearance either as with the bags dumped and my rail rate down it would probably hit the ground.

Other than getting up hills I actually DO like crusing at the lower RPM but mileage and drivability says otherwise.

Changing the steers out is required to keep your front alignment right. If you lower the back end you will change the caster angle of the front. You could get it shimmed to correct that but...the frame rake or angle will be wrong. Most KW run with a negative rake (nose up) but if you exceed the designed amount you can run into some really weird suspension issues. On a low miler it's no big deal but as an expediter you are not a low miler.

I used Eaton's on line calculator to get the results I did, I apologize for not figuring it out by hand (the Eaton site rounds things funny sometimes).

If you call Eaton (number is on roadranger.com) they can guide you through what options are available or give you the name of a local rep to talk to. Eaton folks are pretty helpful. Be mindful when you do buy tires, there can be a fairly large range of revs per mile for a given size depending on the tread and make.

 

[Dynamite 1]

i think the tqe. ratings are between 870 and 1000 or 1050. the one i had,, was a 6spd. autoshift. had 3.73 rears and turned around 17-1750 @ 68/69 mph . i or the trk would usually drop a gear going up a hill my speed always dropped off to 55/60 but with the downshift the rpm is in the sweet spot again which i was told was 1650 to 1950. put alot of miles on it out west. my preference is with a lower gear and use the trans. for keeping the eng. in the rpm range. my current trk has 3.73 gears and gets 9.5 all day long and its heavey 23500#. see what the program shows for a 3.73 or a 3.91 @ 68 and @55/60 in 5th. 55/60 going up hill for a little trk w/a load on consistantly is pretty good sure you get 4th sometimes but speed is still close to 40/45. never overun the engine and the 3126 can have a good life. good luck.

 

[guido4475]

I don't mean to offend you, and please forgive me if i do,But why did you buy this truck to begin with?A test drive would of shown you that it wasn't set up to your liking.But kenworth is an excellent truck.I've seen a few with cummins in them.Just the fact that it has a 3126 in it would be enough to scare me away.It's a non-sleeve engine, a history of dropped/snapped valves due to the lack of valve adjustment intervals.The only way mine has lasted the 828,000 miles was to listen to Tony at cat in richfield, oh. When I first brought it in , he asked me if I wanted it to last forever, do what he says.Every 40,000 the valves got adjusted by him only.He must of done something right, because I have freinds who couldnt get one to last past 250,000.

 

[P51bombay]

None taken, I went with it becuase overall it had most of what I was looking for - KW, 24' box, lift gate, relatively low miles, 10spd and was reasonably well maintained/not abused and with the dollar where it is/was (bought in Canada) the price was right. Ideally I'd have liked a Cummins for the reasons you mentioned but my last truck had a 3126 also and other than having a self changing oil system, aka:leaks - it was a good reliable engine that never once let me down in 2 years. That said a valve adjustment will be next on the list of to do's. Although I have and will spend some money on it addressing some relatively minor issues I expect once those are taken care of it will just need routine maintenance every now and then same as the last one did when a few things were sorted out. Now I could do like the other trucks owner and just run it/fix stuff when it breaks but I'm just not that way............that method came back to bite him on a number of occassions.

 

[bluejaybee]

I don't mean to offend you, and please forgive me if i do,But why did you buy this truck to begin with?A test drive would of shown you that it wasn't set up to your liking.But kenworth is an excellent truck.I've seen a few with cummins in them.Just the fact that it has a 3126 in it would be enough to scare me away.It's a non-sleeve engine, a history of dropped/snapped valves due to the lack of valve adjustment intervals.The only way mine has lasted the 828,000 miles was to listen to Tony at cat in richfield, oh. When I first brought it in , he asked me if I wanted it to last forever, do what he says.Every 40,000 the valves got adjusted by him only.He must of done something right, because I have freinds who couldnt get one to last past 250,000.

Ford and Chevy, Cat and Cummins. The "which is better" lives on. I have never owned a Cummins, but always had 3126's and liked them. If I had had a bad experience with one of them, my story might be different. Maybe it is all in the luck of the draw when it comes to such things. But I would definetiely not own a Ford. Ha!