Had the diff swap done...

[thrash]

I've got an 86 S1853, DT466, and a Thomas conventional 77 passenger body.

It took a big pile of internet research and cleaning and searching on obscure numbers, but I figured out that i have a Spicer CM5552-D 5 speed manual gearbox, and a J190 rear axle. The rear axle ratio was 4.78, and in 5th gear I was showing around 44 mph @ 2000 rpms, and I was gear limited to 70mph @ 3000 rpm. All speeds GPS verified. I am running a 10R20 DRW setup.

My near tires were nearly new when I bought the bus, and I wanted a much lower cruising speed. 2000 rpm is a much better spot for thie DT466 to run at than 3000. I was sick of 1st gear only being good to get the nose of the bus out into the intersection.

So after I found the axle housing part # on the axle (R44CH100), that let me figure out it was a J190 unit. With a little more digging I found the complete range of axle ratios for the J190, and 3.54 was the lowest it got. I did some math and figured out that if I dropped in a 3.54, i could make 60mph at 1900 rpm. That would be just about perfect for highway cruising. I spent a bunch of time with excel and found that for my gear ratios, final ratio, and tire size, if i did the 3.54 swap I could put equivalent or greater torque to the ground at every road speed by using a lower gear, if needed, but that my 5th gear would be so much nicer for the highway.

So yesterday I went and had it done. Catco had no problem finding a 3.54 gearset and they put it in in a day. I have to say that the bus actually is much nicer to drive around town because I don't have to pull the lever so darn often when I'm trying to do other things (like make a left turn without clipping something!).

I hope to do some measured speed vs. rpm comparisons in the near future, and report back on fuel economy differences. Before I recorded 6.6 MPH, holding the engine at constant 3000 rpm / 70MPH, throttle buried (for about 140 miles of travel)

My out the door cost was $2400 to get a totally new diff with a 1 year warranty professionally installed, with 28 qts (!!) of synthetic gear lube. That's more than I paid for the bus, but hopefully I make it back in marginally better fuel economy, less engine noise, and less engine wear. I'm hoping I can cruise in the 60-70mph range and still see 8+ MPG. We'll see.

[chev49]

Thanks for the post. also post when you figure out the new mileage.

[ns9987]

Very Neat, I must say!! I would def. love to see some pics of your 77 pass. Thomas!! That's one of my dream buses. I have an IH 1987 S-1700 Bluebird 65 pass. with a 6.9L engine and an Allison AT-545 Auto trans and it also has Air Brakes, and I run 11R22.5 tires(which are nearly the same specs as a 10.00R20). Now, I have a 5.38 axle ratio, and with 4th in my tranny being 1:1, I can get around at 45mph @ 2500RPMs. lol Now, I know that my 6.9L has nothing on your DT-466, But, you are absolutely right on the cruising speed of your engine. Your DT is an Inline-six, (just in case you didn't know lol) and my 6.9L a V8. Your DT also has a turbo, and my 6.9L, does not. So, what does this mean for us? It means, that because of the engine configurations we both have, we have to pick ratios that our engines work best at. Being that inline-six diesels produce their peak torque at lower RPMs than V8's do, you can choose a higher ratio, whereas ME lol, 4.78 might be all my little 6.9L can handle. lol Now, doing some calcuations here, I figured up you can do about 70mph, at 2000RPMs, in 5th. That's faster than hell!!! lol But also you can get away with running high ratios too, because you have a manual trans, that has some deep gears to get you started. And I only have 4 marginal ones. lol The only thing I can maybe worry about is hill climbing power. The sweet running range of your DT is between 1500 and 2200RPMs. I can't really say that 3.54 is too high, but I guess if you're traveling on the road ALOT I guess it's fine. =) I would LOVE to try your old 4.78 in my bus, but like I said I don't think it would like it too well. lol

NOW!!! Getting into another world, I Also have a 97' Thomas MVP-ER 72-pass, with a CAT 3116 and an Allison MD3060 5 speed push button shifter. Now, even that I said Inline-six diesels make their peak torque lower, it still has a 6.14 rearend in it. BUT, my MD3060 has an Overdrive gear .75, I still can only do 55 @ 2150RPMS though. lol

At any rate, Great job!!! Oh yeah, and remember.... no matter how much money you put into a bus, you'll never get it back. I'm about 10,000 in debt to both of mine lol =)

[opus]

Originally Posted by ns9987

Oh yeah, and remember.... no matter how much money you put into a bus, you'll never get it back. I'm about 10,000 in debt to both of mine lol =)

Thats the best truth I have ever heard here!

[ns9987]

Hahaha!! =D Yeah, that's def. the sad truth about these old buses.

[Will]

School busses are geared to go 50mph. Past that, you drive right out of your power band. My bus, you shift into 6th at 45mph. It has a top speed of 70mph (GPS) which means when you hit a hill, you have no gear to shift down into until you get below 45mph. I've been all over in it but we're going to head out west and I'm not going to sit there buzzing the motor at 2,900rpm across South Dakota so I am swapping out the chunk for a 3.73. $850 for a used chunk is pretty good, and cheaper than getting gears set up.

There is no special reason an in-line motor makes it's power at lower rpms than a V motor. The pistons don't know they are laying on their sides. Stroke length matters as with a longer stroke the piston is moving faster and the air is moving faster at lower rpms, meaning a better cylinder filling is possible. With a turbocharger, that isn't as important. If I had a 6.9 I would get a Banks turbo kit for a Ford pickup. Adding the turbo will make it a totally different motor.

[frank-id]

After the new found road speeds, more engine power will be required. Your 466 is a favorite for me. The 7.3 IDI is OK too. The new power can be derived from a turbo, water/alcohol, LPG or nitros oxide. I changed my Thomas bus rear axle gears from a 6:25 to a 4:10, and now cruise at 68mph. My Thomas is powered by a Kitty 3208, which is very powerless. My trans is a Allison 645 and great to drive. Thomas builds a great bus. Frank

[frank-id]

Diesel engines operate at different speeds. There are 3 different maximium rpms. There are generators and some equipment that operate at 800-1200 rpm. Medium speed diesels operate up to 2500 rpms. High speed diesel engines operate up to 4000 rpms. All the Ford/International 6.0 thru 7.5,, Cummins 5.9, and 8.3, and the GMC diesels, all turn about 3600-4000 rpms. All the competition engines can turn up 12,000 rpm. Each engine maker provides a guide for their operating limits. Many Skoolies turn their engines up to 2500 rpm. RPM does not represent power. There are many add on devices for more power. The easiest power gain for lowest dollar is a turbo. Usually a clean 25-30% power gain with the same engine speed. Frank

[thrash]

Ok, sorry for the late follow up on this.

We travelled from ND to California and Back, going through Yosemitie, the Rockies, etc.

Average MPG on the trip (which was many thousand miles over 3 weeks): 9.8

I was _floored_ at how good the mileage was.

We had as low as 6 and as high at 14, depending on the driving segment.

We saw speeds of 90mph, GPS verified (downhill in Wyo, in a posted-75 zone )

Problem is, going uphill on steep mountain grades, i'd loose too much speed doing the 2->3 shift, and I'd start losing speed in 3rd gear. This limited me to 25mph going up long uphills. That sucked.

But the cruising speeds were great.

[Stuff]

yup. thats the joy of a low gear ratio. but like you said, take your time and the cruising speed pays off in the long term!

[frank-id]

Rear axle ratios are easy to understand. A high number means a low ratio, as 6:25 to 1, as this is a very low ratio.
School buses usually are ordered with a 5:60 to a 4:10. These are gears and speeds for a very controlled school route.
A low number is a high ratio, as 3:73, or 3:55 to 1. Many school bus conversions usually change gears for better miles per gallon and to reduce engine speed. Selecting a low number rear gears, will require some planning as usually the
number will require more power. Too high a rear ratio will make a huge load for a low power engine. A compromise is to install an overdrive unit between the transmission and rear axle. Frank

[100MPG]

Originally Posted by thrash

Ok, sorry for the late follow up on this.

We travelled from ND to California and Back, going through Yosemitie, the Rockies, etc.

Average MPG on the trip (which was many thousand miles over 3 weeks): 9.8

I was _floored_ at how good the mileage was.

We had as low as 6 and as high at 14, depending on the driving segment.

That is fantastic mpg's! Glad to hear it does have an affect. Do you know what the average MPH was? 65? 75? 55? I once drove a GASOLINE RV (ford 460CI) across country at 52mph, BOTH WAYS, in order to save gas. I still only averaged 8-9 due to running the generator. It is still better then the 6MPG on previous trips when I did the speed limits! At 75mph, it was like throwing 50$ bills out the window every 50 miles.

[chev49]

Not all busses are geared for top speed of 58 or so. Mine was bought to take the teams to events, hence the compartments and over 70 mph top speed. My bus can average 10mpg if i am not pulling a trailer.

[Jarlaxle]

Same here: I think I have 4.11 gears...with 11R22.5 tires and a lockup Allison, 60MPH is only 1900RPM.

[thrash]

Prior to swap:

Vmax was 70mph (GPS confirmed), 3000 rpm indicated, in 5th gear, yielding 6.6 MPG

After swap:

Vmax is 90mph (GPS confirmed), i beleive at 3000 rpm indicated, still in 5th gear.

We never intentionally lowered our cruising speed to save fuel. Sometimes the terrain didn't allow us to run as fast as we wanted, in which case we were running the bus at WOT to maintain what speeds we could.

When the terrain/winds weren't limiting our speeds, we were generally running at posted limit +10mph, except when that would have exceeded 75mph, which IIRC is the rated speed of the tires. We would exceed 75 for a few minutes at a time (for instance, to verify the Vmax) when the terrain allowed, but generally our cruising speed was between 70 and 75.

[Will]

I swapped my rear diff as well. I went from a 4.78 (69mph at fuel-stop 2,900rpm) to a 3.73 (88mph at fuel-stop). I haven't driven it 88mph, which there is no way it has the power to do, but it makes 75 no problem. The best thing is, with a 6-speed, I can now use 1st gear and don't shift into 6th until almost 65mph, and have a gear to go down into on the hills. Before, you couldn't downshift until you got below 50mph, which really, really, sux. I now cruise at 68mph and it's a lot quieter. before, I couldn't stand it past 60mph. I can roll now.

I bought a used drop-out section from a truck salvage yard for $800 and changed the pinion seal. Lower numerical gears aren't common in these axles--mine is an N175. Took a lot of calling and I had to drive 3 hours each way.

To handle the diff, I made a mount for it that bolted to the diff and welded the mount to my floor jack (it's a big 100# jack, not a little one--you need wide wheels for stability) If you do it where the bus is parked, and right on the old diff, the new one will line right up and go back in. That phawker is at least 300#, maybe #350.