Hi guys. I have a short bus skoolie with a 7.3L IDI and an AT545. I'm trying to get it to go at least 75MPH. The Spicer f155S is now obsolete, so I'll be replacing that axel with a Spicer S150S which is still available.

I'm not a mechanic, and I only know basic, and maybe intermediate stuff about how a bus works, but I'm certainly no expert. That's why I've called around, asked mechanics, and got information on various topics on this project so far. So, now I'm trying to understand the gear ratios, and how best to select one that will work for my application. My skoolie will be OTR, so yes I'll have some hills, and yes it's going to do a lot of highway driving. Both things I need to account for.

I've heard from Inland Automotive that they suggest using gears in the 3.x range, and when I used the JEGS calculator, it makes sense to do that. However, even if they made a gear ratio of 1, which AFAIK they don't, then it'll lose nearly all the torque, and so it'll have a very hard time with hills and also it'll take forever to get it going.

My current axel is geared at 5.57. As a school bus driving around town, that makes sense, but it'll be ungodly slow if I leave it at that, so I need to change it.

Also, I've read that the 7.3L IDI is often coupled with a 4.x gear ratio in pickup trucks. Presumably it's not in the 3.x range because pickup trucks are usually meant for towing. I don't need to tow anything. I just have a heavy-*** bus. (21,500 GVWR)

Now, I've played with the calculations, and from what I can tell, under load, a 7.3L will probably do 3,000 RPM max, but I've heard some people got it to 3,750 RPM. Regardless, I don't want to always high rev, and I've been counting mine as 3,000 RPM to get TO the top speed. (Probably will be a little slower once it's there and the weight of the bus is no longer as important in the speed calculations, and it's just the wind and road friction. So, maybe 2,500 to maintain the top speed? I'm not a physics expert, so I could be wrong.)

If I run the numbers, I can put in a 3.21 gear ratio (better MPG, hopefully better MPH, but lower torque) and a 3,000 RPM, my tire size of 22.5, and that gives me a top speed of 63MPH. Still too slow IMO, but better than the 35MPH original calculated top speed.

Also, if I do a 3.9 gear ratio, I get a top speed of only 52MPH. That's worse. I don't think there's anything lower than a 3.21 gear ratio for the S150S Spicer axel, and even if there was, I don't know if the engine would be able to drive that or not.

While I REALLY don't want to drop $8,000 on a new transmission, parts and labor, I'm open to that later on if I can't be comfortable with just replacing the axel to get a better gear ratio. However, I'm not sure what good that'll do, but it might improve the torque.

What do you think is the best course of action to get to 75MPH+? What gear ratio do you recommend and why? I'm not willing to buy a new engine again, as I'm already having the broken one replaced for $8,000 parts and labor. (Jasper reman) Plus, a cat engine costs even more, and the mechanic said it's the only one that could replace it because of the frame carrying capacity. Anyway, no engine change. I'm willing to sacrifice torque to the point of a slow speedup, as long as I can maintain a good top speed, but still climb steep roads at 40MPH+.

I appreciate your help on this. Remember, I'm not an expert, so just educate me if I'm way off base. I've already built out the skoolie for $70k and 9 months of work, so I'm not swapping busses either.

EDIT:
Hi guys. I think after hours of working on this, I've learned why the MPH was so low on the JEGS calculator. The 9R22.5 size is 22.5 INNER diameter, and since the OUTER diameter meets the road, and thus is used in speed calculations, I should have used the outside diameter of 38.30. As I'll be spending about $2k on the axel swap, I'd still like to know which ratio will be better for my use case though. Now that I know the speeds are there, I'll still need to know how to maximize the torque. Please help me choose.

 

545 is 1:1 high, no lockup. It will always hinder your progress.
https://www.skoolie.net/forums/f13/h...ios-23065.html

If you stay with a 1:1, this would apply:

38" tire, your current 5.57 ratio and 1:1 transmission means 3,694 RPM at 75 MPH.
38" tire, 3.9 ratio, 1:1 trans = 2586 RPM at 75 MPH.

AT2000 is probably your best answer.

https://www.skoolie.net/forums/f37/a...643-14661.html

Under $1000 for the most part:

https://truckpartsinventory.com/part...assys-for-sale

 

Thank you. I've been looking at the Allison 2000 tonight. Do you know if it's a direct replacement, or if it'll need some kind of adapters? I know it'll need a different shifter, but beyond that I couldn't find anything telling me if there's a necessary adapter. I planned to call Inland Automotive on Monday about that.

​​

 

I think your 7.3 will have an SAE pattern. The Allison will have an SAE pattern. So you may have a direct bolt in.

Beyond that I'm guessing.

I do think the transmission swap will be better overall. You don't have to fool with your brakes, and you get more ratios at the lower speeds which will make it run and drive a lot easier IMO.

 

Mine is a 94 model, and it has a driveline parking brake, and the AT545 doesn't use a parking pawl. I'd like to keep my driveline brake, and also use the potentially included Allison 2000 parking pawl. Will that mess things up, or will I need to choose one over the other? I know you said I don't need to mess with the brakes, but I just want to confirm how that'll work.

Also, since my bus doesn't have an ECU, will the TCU still work on its own?

 

Can't even guess about that. You'll have to sort it out unless more qualified help arrives here.

That skoolie site might could have more particulars........

 

I've decided to go with the MT643. I found one with the parking brake module in case the one I have can't move to the new tranny. That way I have a driveline parking brake. (Round disk at the back of the transmission is the driveline brake)

Plus, it's fully mechanical and so it doesn't need an ECU. It locks in gears 3 and 4 which is the whole point of the replacement. Plus, it's compatible with the 94 7.3l IDI, but the engine uses SAE #3, as does the AT545, but the MT643 uses SAE #2. So I'll probably need an adapter.

I've also heard that it might be a bit longer than the AT545, but since the engine will be out when the tranny gets changed (Engine is getting replaced, too) then the mechanic might be able to move the engine up, or the tranny back. Plus, the rear axel is getting replaced, so there's room on that end, too.

Expensive. ***. Drivetrain. Replacement. Lol
​
I appreciate your help on this. For anyone who's curious as to what gear ratio I'm going with for this, I'll go with the 3.21 for more flexibility.

Check out the MT643 specs at https://www.wanderlodgeownersgroup.c...43%20SPECS.htm

 

OK that's still a 1:1 direct transmission so you are gaining only the TC lock which won't amount to much with respect to speed and rpm......

 

If I plug in the numbers at https://www.jegs.com/tech-articles/g...las-resources/ then I'll see that a 3.21 ratio, 38.30 tires, and 2500 RPM will result in a top speed of about 89MPH. So that solved the speed issue. However, without a locking transmission, it'll have a hard time with incline roads. So it needs torque.

Am I missing something? I mean, a 1:1 4th gear will mean the only limiting factor is the 3.21 gears in the differential. Everything else is the raw output of the engine. Why is that a bad thing? Just trying to understand.

EDIT:
I did a bit more research to understand what the mod said, and here's what I've learned. While the 3.21 is not a 1:1, and will offer more torque, the MT643 is usually coupled with a 4.1 to give it more get-up-and-go. So, if I want it to go fast, but speed up faster, I can use a 3.92 for the best of both worlds. It'll still achieve 87MPH at 3000rpm, as opposed to the 106mph I'll never use at 3000rpm. This will keep it closer to the typical ratio for torque, still give me the speed, and the locking will allow the RPMs to be lower due to locking, thus avoiding transmission burnout.

As far as I can tell, this is a good setup for mountains and for highway speeds on my short bus skoolie.

 

instead of messing with the trans, i would look into an overdrive unit like a gear venders, or a two speed rear.
dropping down to a 3.XX rear gear ratio with a 1:1 final drive in the trans on a 7.3 NA engine will take 2 years to get up to speed.

 

If it takes to long to get going in 4th gear right away, I have no problem starting in 1st, then shifting my way up the whole way. Only 4th is 1:1.

I don't want this to be overly complex. I've already spent $8k on a replacement engine since the last one was busted. Now I'll be spending about $8k more for the parts and labor on a rear axel and tranny. Way too expensive already.

 

A 5 or 6 speed transmission with OD would help your case.

Another 4 speed with 1:1 4th would, imo, be a waste of time and money.

With the OD you can keep your axle, which will help you out at lower speeds.

I already said this above--then you switched to going to another 4 speed.

I think you need to draw this out on a piece of paper and think about it some more.

The transmissions I found were not much over $1000. Installation will cost, but it won't be near $7000.

Or just drive it like it is and save all that money.

 

Given that I don't have an ECU, and I want to stay fully mechanical, the 2000 series is out. The only other popular option is the MT643. I need to replace the axel to change it out so it's not 5.57, and they don't make gears for the f155s that's in there right now. If the gears wind up not being to my liking, I could always swap those out and sell the one I'm starting with. (I'll be selling the AT545 and the old axel anyway.)

The $8k was actually supposed to be $5k. I forgot I was budgeting the $3k labor cost for the engine replacement in that. So $5k covers the axel, transmission, and labor.

I still don't see what's wrong with a 1:1 on 4th. AFAIK, the only thing that means is the engine is directly driving the rears, and with locking, the transmission will just clutch the engine spin onto the shaft in the transmission to send that power to the drive shaft without extra gears getting in the way like without a locking/clutch torque converter between the transmission and engine.

 

The one thing you are not taking into consideration and that is HP. You are pushing a barn door down the road with lots of wind resistance, you do not have enough HP to take you where you want to go speed wise IMHO.

I worked in that transportation industry for many years. Normally a bus would be gear for 65 mph top end if doing much over road stuff and then it had a DT466 at 250 HP to move it if your talking a Navistar product. Put a large hill in front of you and all bets are off. Seven pre cent grade will have you down to 40 maybe 45 and that would be 250 HP connected to a MT643. Been to many years to tell you a common rear ratio but I would guess low 5's.

That sort of combination would turn over at 2600 rpm which would be governed speed for a 466 and if lucky you will get around 6 mpg.

Your money but I sure would not be spending it trying to get high road speed out of a IDI especially in stock form as I do not think you will ever be happy. Not moving a barn door down the road.

Why not drop a Road ranger 13 speed into the thing. You can get OD and if your doing much hill climbing find a gear more closely match to where you want to be at. They did put manual transmissions in buses and I am guess you have a S series chassis based on your engine. The school bus industry did not use that sort of manual due to the fact you can get them hung up out of gear due to them not being synchronized. The other issue you would have to work around would be air for the range shifts.

 

IHC1470, you just beat me at it.
What I read here is, (No pun intended) someone who likes scoolies, and wants to end up doing everything with his, still OK.
then invests quite some in having made it nice, and then adapt it's mechanics to do what he wants, sill OK
but then, having a 21,500# bus with the nice curves of a "brick" going down the highway at 89 Miles/H, and all that with the modest 7,3 IDI???

In my opinion not really realistic, not even if you invested a million dollar in it.
"Horses for causes" comes to my mind.

As I wrote, post starter, don't forget that a scoolie has the aerodynamics of a brick, and you want it to speed it like a formule 1 car, one must not forget that wind resistance quadruples when you get above +/- 55 miles, that means fuel consumption skyrockets as well.
And aside of that, the 7.3 is just not capable to do this, it just hasn't the "ooommpf" for it. It has already a slight difficulty with a Max gvw 11.000# F-350 up a hill when it's empty.

I think, (and I don't wannabee a hater or something like that) that you might be better off when you adjust your goals for your scoolie, and accept that certain vehicles just drive the way they do, and that there is a reason why they do so.

further all the best with your project, and I really like to see a Image of it, it seems you put a effort in it?
Be wise, good luck, I follow, it interests me.