Rear differential advice

[LittleBusBigDreams]

Here comes another "T444E / AT545 need lower RPM's on the highway" post


1997 Thomas Vista 3600


I had the local International parts guy pull the rear end info using my VIN and was told it's a Dana model 15500 (can't find this model mentioned anywhere?) with a 3.39 ratio. My tires are 245 / 70 R 19.5 and measure 32.25" in diameter.



When using an online calculator with 3.39 gear ratio it calculates very close to my actuals:
50 mph @ 1866 RPM (80 kph)
55 mph @ 2052 RPM (89 kph)
60 mph @ 2239 RPM (96 kph)


When I spoke to a local drivetrain shop last summer they mentioned a 3.2 ratio would be the next step, which only drop ~100 RPM at highway speed:
50 mph @ 1761 RPM (80 kph)
55 mph @ 1937 RPM (89 kph)
60 mph @ 2113 RPM (96 kph)


I would love to cruise at 60 mph around 2000 RPM, which would land me somewhere around 3.0.


This leads me to some questions:
1) Is 2000 RPM a reasonable cruising RPM? From what I know it's in the peak power band, and should be more reasonable on fuel vs my current 2300 RPM cruising speed.


2) 3.0 seems very low for a skoolie. From what I've read most busses seem to be in the high 3's or low 4's. Is 3.0 recommended or even achievable?


3) I already slow down significantly on the hills, and I believe i'm usually in 3rd gear doing 30 mph at the end of a long hill. At 3.0 I realize I'll have less pull up the hills, but am I naive to think I'd just end up in 2nd gear doing 30 mph?


4) If I end up tinkering with the rear end, I imagine I should try to end up with a posi / limited slip setup. Plan to spend a lot of time on logging roads and some extra traction seems like a good idea. If you have experience/advice in this regard it would be appreciated.




Thanks in advance for any input!

 

[Booyah45828]

https://www.skoolie.net/forums/f37/transmission-options-29535.html

Rolesville swapped his trans, gained an od, and got the bus he wanted. He did most of the work himself, and I believe it cost him very little in the end.

If your bus hasn't been yet converted, I'd suggest buying a different bus with the gearing that you want, over spending thousands plus the cost of the bus to get you that. I'd also advise against logging roads, as they're pretty rough and I'm not sure if a school bus would be durable enough.

 

[LittleBusBigDreams]

I a lot invested in this bus so far, so need to stick with this chassis.


I need new tires anyway, and since my gear ratio is already so low I'm looking into changing from 19.5's to 22.5's.


Is this something people do? Anyone have experience in this regard?

 

[Ronnie-SKO]

Measure the wheel well opening, most likely 22.5's should fit. A 10R22.5 is right about 40 " diameter. This would help the gear ratio. Although 2200 rpms at 60 is a nice rpm for the 444. They do not lug down as low as a straight 6, like the DT466.



What are you hoping for as a cruising speed on the highway?



I do have a Detroit locker in my rear, and it really is great off road. Especially when you get a tire off the ground in an off camber situation. Big tires and low gearing(high numerically) are best for off road. Lets you get the speed down so that you can gently drive rather then blasting through Baja style. My crawl ratio is 50:1 . Manual trans with granny gear and a two speed rear in low. I have seen to many buses just stall out in soft sand and not have the gearing to even turn the wheels. This is with the AT545 trans. Fire roads tend to be better then logging roads. I do think you would have trouble on a logging road with your current set up.


Must say I like the looks of the Vista buses.

 

[La Camioneta]

How mobile do you plan to be? Touring or just moving occasionally?

I did the A2000 swap behind a 12v 5.9 and would do it again in a quick minute. Unbelievable difference in drivability and no need to regear as the OD effectively does that for you.

Changing rear gears isn't a workaround for a 1:1 non-lockup transmission. You'll suffer the same converter slip and poor drivability.

If your current state is intolerable, do the Allison swap. Otherwise, tall tires and deal with it.

 

[LittleBusBigDreams]

Measure the wheel well opening, most likely 22.5's should fit. A 10R22.5 is right about 40 " diameter. This would help the gear ratio. Although 2200 rpms at 60 is a nice rpm for the 444. They do not lug down as low as a straight 6, like the DT466.

What are you hoping for as a cruising speed on the highway?

I do have a Detroit locker in my rear, and it really is great off road. Especially when you get a tire off the ground in an off camber situation. Big tires and low gearing(high numerically) are best for off road. Lets you get the speed down so that you can gently drive rather then blasting through Baja style. My crawl ratio is 50:1 . Manual trans with granny gear and a two speed rear in low. I have seen to many buses just stall out in soft sand and not have the gearing to even turn the wheels. This is with the AT545 trans. Fire roads tend to be better then logging roads. I do think you would have trouble on a logging road with your current set up.

Must say I like the looks of the Vista buses.

I'm happy to cruise at 60mph, but hoping to do so closer to 2000 rpm vs 2300 rpm. My redline is set for 2500, and running at 2300 all day seems a bit tough on the engine and i notice a fuel economy hit.


By my calculations I should drop around 300 RPM by moving from 245/70R19.5's to 10R22.5's.


It's a 1997, the last year without a TCM, so hopefully my speedometer adjustment is fairly simple for the tire shop to handle.


Thanks for the feedback on the rear end - i'd love to move to a locker but that's a project for another time i feel.

 

[LittleBusBigDreams]

How mobile do you plan to be? Touring or just moving occasionally?

I did the A2000 swap behind a 12v 5.9 and would do it again in a quick minute. Unbelievable difference in drivability and no need to regear as the OD effectively does that for you.

Changing rear gears isn't a workaround for a 1:1 non-lockup transmission. You'll suffer the same converter slip and poor drivability.

If your current state is intolerable, do the Allison swap. Otherwise, tall tires and deal with it.

I'd love to do an A2000 or MT643 swap but don't have the time/facilities/skills right now to tackle that project.


Appreciate the comment regarding the rear gears - I had a feeling that might be an issue. Hopefully taller tires won't also aggravate that issue.



I'm trying to figure out what my slippage currently is related to RPM, and at highway speeds I *seem* to be losing around 100 RPM vs what the calculators are telling me I should be at. Not sure if this is normal with a AT545 or not.

 

[Ronnie-SKO]

Yes a hundred or 2 rpms extra for torque converter slippage is normal for the at 545. The steeper the gears the more slippage as well, in a heavy vehicle.

 

[EastCoastCB]

I'm happy to cruise at 60mph, but hoping to do so closer to 2000 rpm vs 2300 rpm. My redline is set for 2500, and running at 2300 all day seems a bit tough on the engine and i notice a fuel economy hit.


By my calculations I should drop around 300 RPM by moving from 245/70R19.5's to 10R22.5's.


It's a 1997, the last year without a TCM, so hopefully my speedometer adjustment is fairly simple for the tire shop to handle.


Thanks for the feedback on the rear end - i'd love to move to a locker but that's a project for another time i feel.

I think CadillacKid did a thread on here about adjusting the speedometer on an IC cluster.

 

[La Camioneta]

I'll beat this dead horse once more but the A2000 is a bolt-in swap, physically. They're abundant in the unloved 6.0 PSD busses that get scrapped rather than resold.

You may need a different stall converter.

Cadillac Kid can help you with the electronic end of it, its not exactly plug and play but its by no means impossible.

A good heavy truck shop can do the swap for you and possibly the TCM integration.

Best bang for your buck IMHO.

 

[Ronnie-SKO]

The 2000 Allison with 6 speeds would give the range to allow a reasonably slow speed for logging roads, and still allow good highway speed. Hexwould most likely need to change rear-end gears to get the sweet spot. Thing is off road and good highway are kind of opposite in gearing needs, so the more gears the better. With a 5 and two that gives me 10 speeds to work with. Our new bus has 6 speeds, but I have no intention of off road with it.