SKOOLIE - Governed or Geared?

[heathnlaura]

First let me thank whoever made this forum. My better half and I had been kicking around the idea of tent camping alot with the kids, then trailer camping, then decided RV camping would be the way to go. We always thought, "gosh, if that were there it would be better, or maybe if that were facing the other way". I stumbled upon this guys site with a nice looking green diesel pusher and we knew that was the way to go. In looking more, we found this site and there is no turning back now.

About a month ago we headed up to Reno and came back with a 91 International Blue Bird. She's got a DT466 mated to a MT643 with 168k on the odometer. Originally a shcool bus in Eugene, Oregon, she got a paint job and hauled fire crews out into the woods for a few years in Oregon, the the guy with a party bus business thought it best to buy this bus to haul hippies to the outdoor festivals instead of his leather clad bus. He decided to close shop and now she's all ours. We've got all the seats out, rust done away with via an angle grinder and wire brush, then coated twice with Rustoleum paint for aluminum and galvinized steel, then coated again the the bullseye 123 primer (also made by Rustoleum and good for aluminum and galvinized steel) with Insuladd ceramic beads for insulation during the cold. We'll be putting up pictures soon.

My question though is with the top speed. She did very well in the mountains. Only slowed to under 55 when I got stuck behind a really slow truck or going around some of the turns as the road was wet and it actually started to snow a bit even though the day before when I went to see the bus it was 85 degrees. On flat ground, she would only do about 63. I still had a little pedal left, but laying all the way into it didn't change anything. Or if it did, I didnt have the 1k mile straight away needed to realize the oh so slight increases in speed. How can I tell if this is due to a governer on the motor or the gearing? I have no idea what the gears are the the pumpkin and probably wouldn't change anything until we get to traveling through the prairies of Kansas or something like that. I like taking my time and enjoying the scenery and all, but I'd at least like to to do the speed limit.

Any thoughts would be greatly appreciated.

Thanks in advance,

[Redbear]

Regarding the governor, look for posts from member "Nevrenufhp" for making adjustments. Look for other posts for axle ratio versus tire size for top speed information. Welcome, and Good luck.

[100MPG]

Originally Posted by heathnlaura

How can I tell if this is due to a governer on the motor or the gearing?
Any thoughts would be greatly appreciated.
Thanks in advance,

If it is the governor, the RPM'S will be lower when in top gear then they would be in neutral or any other gear when your foot is to the floor. While parked, nail the throttle. Take note of the max rpm's and see if it is the same while cruising down the road with the throttle nailed to the floor. Chances are though, if it takes a while to get to top speed in top gear, it is just the lack of power and gearing.
If it is the same, you will save a ton of fuel and get better mpg's doing 50mph instead of 63. The question is, can you do it with out going nuts!
I drove cross country in a motor home and did 52 mph all the way across country and back. THAT, was a long drive...but I had to do it.

[lapeer20m]

I have found that driving 60 instead of 65 saves a ton of fuel on a cross country trip.

My guess is that you are governed at 63mph. A dt466 with lock up torque converter and the correct rear end ratio you should be able to achieve 80 mph on the flats. You likely have a terrible rear end ratio, and the engine is likely governed on the low side.

There are two adjustments you can make for free: increase fuel (hp) and increased engine speed. These adjustments can be made in just a few minutes. You can easily gain 5 mph top speed, and you can improve your hill climbing ability. It is important to keep an eye on your temp gauge while climbing hills.

Ill search for the link....

[lapeer20m]

Found it...

viewtopic.php?f=44&t=588&hilit=horsepower

[opus]

I can do 80 but my wallet cant. 62 is pretty much what I stay at.

[JakeC]

Most of the DT466 were governed around 2300, if I remember correctly. If you can tell me your tire size and what rpm you were at, at 63 mph, I can tell you your gear ratio

Your answer actually hasn't technically been answered correctly, yet. You have a 91 which means non-electronically controlled. There is no way your bus is literally only capable of going 63 mph (if it wasn't governed). What is being limited is the rpms, under load. If your bus is governed at 2300 rpms, then your speed will be limited to whatever your tire speed is, at that engine rpm. Your gear ratio is determining your speed, based on your engine speed, but your engine speed is limited by the governor. The answer to your question? They are working together to limit you.

I have a 4.78 rear ratio and was governed at 2300 when I got my bus. I am guessing 63 was around my max or maybe even a little less. I tweaked my fuel pump and can now hit 2900 at around 70 or a little more. I still cruise at 65ish, normally, due to fuel consumption.

[lornaschinske]

axle ratio versus tire size for top speed information

Is there a way to figure out just what is a good axle ratio? Ours is 5.29. It's not going to be changed. I'm just curious. We've not got a problem with meandering down the primary highways. MPG's are more important to us than MPH.

Documentation on schoolbuses suck! I don't even know what make our rear is (plate inside says R125NX5). "R" = Rockwell?

[Motobus]

Jack one rear wheel up, put it in neutral. Turn the wheel one revolution and count how many times the drive shaft spins. For example, if the drive shaft turns about 5 1/4 turns for every spin of the wheel you have 5.29's.

[JakeC]

First, it depends on the motor. I am going to use mine as an example. My peak torque is under 2000. I want to be able to cruise at 70, and still be within my powerband for maximum efficiency. I have a non-overdrive transmission, meaning my final gear is a direct 1:1 ratio. If my engine is spinning at 2000 rpm's, my gear ratio is 4.78:1, and my tires are 42", the math would be as follows. 2000/4.78 = 418.42 tire rpms. 418.42*131.88 (tire circumference) = 55181.23 in/min which converts to 52.25 mph. To be in my powerband with my current gearing, I need to drive 52 mph. Not gonna happen. Now if I plug in 3.55 for a gear ratio, the numbers become 2000/3.55 = 563.38 tire rpms. 563.38*131.88 (tire circumference) = 74298.59 in/min which converts to 70.36 mph. That's more like it

I don't know where the powerband is on your motor, but that's how I figured what ratio I want. To be honest, I would be happy with a 3.73, or even a 3.91, as well.

[JakeC]

Originally Posted by Motobus

Jack one rear wheel up, put it in neutral. Turn the wheel one revolution and count how many times the drive shaft spins. For example, if the drive shaft turns about 5 1/4 turns for every spin of the wheel you have 5.29's.

I think she meant how do we figure what ratio we want, not what we already have.

[lornaschinske]

Originally Posted by JakeC

... If my engine is spinning at 2000 rpm's, my gear ratio is 4.78:1, and my tires are 42", the math would be as follows. 2000/4.78 = 418.42 tire rpms. 418.42*131.88 (tire circumference) = 55181.23 in/min which converts to 52.25 mph. To be in my powerband with my current gearing, I need to drive 52 mph. Not gonna happen. Now if I plug in 3.55 for a gear ratio, the numbers become 2000/3.55 = 563.38 tire rpms. 563.38*131.88 (tire circumference) = 74298.59 in/min which converts to 70.36 mph. That's more like it

We do not know what the RPMs are either. I only know the ratio because it's stamped on a plate inside the bus.

[Das Mel]

Originally Posted by lornaschinske

I only know the ratio because it's stamped on a plate inside the bus.

And even then it could still be wrong.

[crazycal]

Originally Posted by Das Mel

And even then it could still be wrong.

Hi Mom, how about a poke in the eye. LOL.

[Das Mel]

Hey, I only poked me eye once!

Okay, twice.

...fine, I admit I did it more than twice I never said I wasn't accident prone. At least I haven't managed to punch myself this week! ...yet. Oh, shut up. :P

[JakeC]

Originally Posted by lornaschinske

We do not know what the RPMs are either. I only know the ratio because it's stamped on a plate inside the bus.

You actually don't need to know what actual rpms you are turning currently. The calculations are all hypothetical I'm surprised you don't have a tachometer, though! Do you know what transmission you have? It can all be calculated from there. If you tell me what speed you like to spend the majority of your time at, what transmission you have, and what size your tires are, I can give you an estimate of an ideal rear gear ratio. I could do a google search of your engine and see where your powerband is. I realize you don't actually want/need to do this, but sometimes it's fun to pretend, lol

[crazycal]

Originally Posted by lornaschinske

Is there a way to figure out just what is a good axle ratio? Ours is 5.29. It's not going to be changed. I'm just curious. We've not got a problem with meandering down the primary highways. MPG's are more important to us than MPH.

Documentation on schoolbuses suck! I don't even know what make our rear is (plate inside says R125NX5). "R" = Rockwell?

You might know this already but the lower the number, the more it is highway gearing. Ideal in a school bus would be in the 4-4.5 range. If you're under 4, you will need a lot more HP to push you down the road or have a tranny with more than 4 gears to find the right sweet spot for your engine so you are not lugging your bus. Also, if you have an AT545, changing anything before you change that tranny is a waste of time and money. These are just my opinions, nothing else.

[JakeC]

Originally Posted by crazycal

You might know this already but the lower the number, the more it is highway gearing. Ideal in a school bus would be in the 4-4.5 range. If you're under 4, you will need a lot more HP to push you down the road or have a tranny with more than 4 gears to find the right sweet spot for your engine so you are not lugging your bus. Also, if you have an AT545, changing anything before you change that tranny is a waste of time and money. These are just my opinions, nothing else.

I realize I may be a little optimistic thinking I can spin 3.55, but keep in mind most of these buses are geared for 55 mph or less. Many geared for rural gravel roads, or cities. When we take them out on the highway, we are spinning them well past the powerband. I do understand what you are saying with needing more gears to find the sweet spot, though. That is a problem for sure if you go with a numerically lower gear ratio. Just need more fuel and boost to bridge the gap

[opus]

Put in 3.55's and throw in a 10 spd Road Ranger as well. Problem solved.

[JakeC]

Originally Posted by opus

Put in 3.55's and throw in a 10 spd Road Ranger as well. Problem solved.

I like your way of thinking Seriously, I would be in heaven.