Gas to a 5.9 Cummins heart transplant

[djones]

Hello All,
Don here with a problem I am having. This is a great site and I have been reading 'till I'm crosseyed. I hope I can get some info even though I have a 36' HMC motor home. It is on a Gillig bus chassis and has a 460 Ford gas engine. We just came back to MT from Vegas country and got 5.5 MPG. Whew! I got on line and bought a 5.9 Cummins with a Allison 545 4 speed. The Cummins is rated at 190 HP.
My problem: At 60 MPH I am turning 2900 RPM with the gas engine. I have 19.5 wheels and tires are 33" tall. I think I have 4:88 gears...I think because no tag on Pumpkin.
Now: I want to turn 1850 at 60 MPH or whatever is recommended. What gear ratio should I look for?
The engine is from a Blue Bird bus with 22.5 tires. I have new 19.5 tires and no room for the larger tires. The 19.5" tires will stay.
The engine swap will be kinda smooth because the Gillig Chassis is 1" wider than the Blue Bird. Just some spacers needed.
I am looking for Blue Bird cross member, rear mounts and a side mounted air compressor with power steering behind that. I am also looking for a A/C pump and mount for the 5.9 if any one out there has a source or extra.
Thank for any information I can get, Don

[Jive Turkey]

The Allison 545 is a non over drive trans mission. In fourth gear the trans is at 1.00:1 or direct drive.

A 33" tire has a circumference of 103.7" That tire will turn 610.99 times per mile. 60 miles would be 36659.595 turns of the tire. 36659.595 turns of the tire in 1 hour divided by 60 minutes is 610.99325.

60 mph or 610.99 tire rpm at 1850 engine rpm's and 1.00:1 out of the trans (assuming the torque converter clutch is locked) you will need a 3.03:1 differential reduction for that to work to your spec's.

If you had 22.5's @ 41.3" diameter you would need a 3.78:1 differential reduction for your spec's

Assuming the trans you have right now is direct drive in top gear then your diff reduction would be 3.74:1 to make you go 2900 rpm's at 60 mph. If you did like one of the other guys did and put a 3.55:1 you could cut you rpm's down to 2169 from 2900 at 60 mph.

[djones]

Hey Jive Turkey, thanks for the great reply. If you were in my shoes what RPM would you shoot for? I assume I will have to change the carrier to go from 4:88 gears to 3:55.I am looking to get the best fuel economy. My transmission is a 545 Allison. I have read where there is an Allison available that locks into 1:1. That would be a fuel savings trans. I wonder if the 545 can be driven in 3rd for any length of time. If so, a guy might be able to go higher yet in the rear end (3:40's)?
Can you remember who the person is with 19.5 wheels?
All the guys with 22.5 wheels.....I wonder what RPM they are looking at for a cruise speed and a 5.9 Cummins engine? As I see it, the school bus is made for the start and stop use and a higher rear end ratio is needed but which one would make a good RPM?
Thanks again for the great reply and I will copy and put in coach, Don

[Jive Turkey]

Originally Posted by djones

Hey Jive Turkey, thanks for the great reply. If you were in my shoes what RPM would you shoot for? I assume I will have to change the carrier to go from 4:88 gears to 3:55.I am looking to get the best fuel economy. My transmission is a 545 Allison. I have read where there is an Allison available that locks into 1:1. That would be a fuel savings trans. I wonder if the 545 can be driven in 3rd for any length of time. If so, a guy might be able to go higher yet in the rear end (3:40's)?
Can you remember who the person is with 19.5 wheels?
All the guys with 22.5 wheels.....I wonder what RPM they are looking at for a cruise speed and a 5.9 Cummins engine? As I see it, the school bus is made for the start and stop use and a higher rear end ratio is needed but which one would make a good RPM?
Thanks again for the great reply and I will copy and put in coach, Don

The AT 545 is 1:1 in fourth gear. Unfortunately your 19.5 wheels aren't helping you with what you are desiring from your bus. I would call your local dealer and find out exactly what gear ratio's are available for your application and go from there. The calculation's are easy to figure what your speed/rpm range is going to be once you have real ratio's to deal with. You may be better off just changing the diff ratio's on what you already have than to change the whole drive train. Personally I think changing the whole drive train to gain a few mpg is a pretty ambitious undertaking. i would try changing the diff gear ratio first then go from there.

[djones]

Thanks again for the reply. I have taken the 460 gas engine out and am getting ready to install the 5.9 Cummins and 545 Allison.

I still have the 4:88 gears to deal with. I had an O/D trans through a Gear Vendor setup. I cannot use it with the 545.

It used to turn 3,ooo RPM in a 1-1 mode at 60MPH. That is how I know with out O/D that will be too low a gear or rearend. I feel the 3:50's gear ratio will be right in there.

Again, the mileage was 5.5 MPG and the gas had to go!

I still would lke to know what RPM to run this Cummins at a cruise speed.
Thanks again, Don

[Jive Turkey]

Originally Posted by djones

I feel the 3:50's gear ratio will be right in there.

I still would lke to know what RPM to run this Cummins at a cruise speed.
Thanks again, Don

A 3.50 will put you at around 2140 rpm at 60mph

I found in the cummins forum a poster mentioned he was doing 63 mph@2000rpm with the 5.9l. Granted that was a pick up truck and not a bus but you would figure that the cruising rpm range should be the same or close to the same.

[kotflb]

With your short tires, you could change the ratio to 2.90 and run 61 mph at 1800 rpm . It really depends on the axle make in order to find out which pumpkin ratios are offered. The Cummins 5.9 gets its best economy between 1700 and 2000 rpm. If your only gonna cruise at 60 mph, I myself, would go for the 2.90 axle ratio, this way, when you hit those small 1 to 3% inclines and your engine speed starts falling, your engine torque will save your transmission from having to downshift. Meaning, you should be able to crest most of those inclines between 1650 and 1800 rpm, depending on how heavy you are of course.

[djones]

Thanks for the great reply's and information. I just found out I have a Spicer/Dana rearend housing D135S and a 5.29 gear ratio. I cannot find a diff. ratio higher than 5.29. They go lower to 7.00 ?? Almost a tractor speed.

I like the 3.25 ratio for what I am looking for in cruising. It looks to me a double O/D unit would be the hot tip. The computer issues are holding me up there.
Any more advice would be appreciated, Don

[Jive Turkey]

What transmission did you have in there before? Check out gear vendors they sell bolt on o/d units but I don't see anything for allison.