Engine,Gear ratios, GVW, mph???input

[JoeA]

I love these forums. I have been contemplating, calculating and bullsh*ting with folks over time and still haven't made the gear change. I just made a post to the fuel mileage forum about this but thought I would ask for some feedback. MORE OPINIons anyway...

I get 8-10mpg regardless of terrain. Usually closer to 8. I have a DT466 that runs great but smells a little rich if you compare it to a diesel Light truck or something. It has 4:78 gear ratio with the Allison 4spd AT545. I have 30" tires(245/75R22.5) I never have my foot to the floor to maintain speed. once its up to speed I actually don't press it all that much, just enough to keep it pushing those low gear ratios. This is why I think 4:10 gears would be fine.
I am hoping that I might get a steady 10mpg with an occasional plus or better(10-12mpg), run quieter at 55mph and get a top speed of 67mph at the same rpms 55mph is now. I would love opinions on this. My GVWT 25,800. I actually weigh 19,500 with full fuel and 55 gal of water and all my belongings(outdoor gear and tools) I live in this thing. The speed to rpm ratios I know exactly because I used a little paper gear dial and checked it to the current tire size and rpms and ratio and everything matched up.

[Les Lampman]

Hey Joe,

Those numbers don't work out quite right; all the charts I found on the 245/75R22.5 tires say they're 37.1" to 37.4" in diameter.

What speeds at what rpm's are you getting now?

This is the online calculator I've used for all my computations:

http://www.csgnetwork.com/rearendgearcalc.html

I have a DT466 in my 1979 Blue Bird full size (66 pax) bus. I drove home from Denver to north of Seattle and made three round trips (1400 miles each) to northern California from NW Washington. Lots of hills on all my trips! On the flats I get just a hair over 9 mpg (usually about 9.2) , in the hills I'll drop into the low 7's. On the trips to California I average about 8.5 mpg.

I run at 55 mph at 2400 rpm. Wide open, foot on the floor, hoping for a tailwind speed is about 57 to 58 mph at 2600 rpm.

If you use 2600 as the rpm, 4.10 for the rear-end, 1.1 for the final gear ratio (allowing 10% slip on the AT545), and 37.1 for the tire diameter that comes out to 63.6 mph. You'd be around 2450 rpm for a 60 mph cruise.

To get to your hoped for numbers it seems you'll have to go a little numerically lower for your rear-end. A ratio of 3.70 would give you 60 mph at 2200 rpm and 70 mph at 2600 rpm (is that your governed rpm?).

What's really hurting you are those itty-bitty tires; do you have room for something taller (like 11R22.5)?

[the_experience03]

With a gear ratio drop, I wouldn't worry so much about power as the longevity of your transmission. The DT466 is about the biggest skoolie engine you're going to find. Your version might be detuned to fit under the 210bhp cap on the AT545, but I'd be willing to bet it's still right at the threshold. The low gears might be what's keeping that tranny together.

I know lots of guys who buy a pickup truck, lift, and throw some 35 inch tall tires under it. They might do some minor engine mods to "get some power back" but leave the ratios in the axles the same. The end result is that there is a greater load on the tranny's output and a greater load on the tranny's input and a year later they're rebuilding the tranny AND swapping ratios. I'd be willing to bet with your current configuration that tranny isn't experiencing much torque converter slip, but with higher highway gears, it might start multiplying. Just some food for thought though. I really don't know for sure. Perhaps a call to an Allison specialist might be a good starting point.

[lapeer20m]

i'm pretty sure i read in an earlier post that he has the mt643?

[JoeA]

Hey thanks for all the input...I'm on vacation right now and will have a chance to read and reply to all the info

[JoeA]

Thanks for the online claculator. I will check it out.
Ummm First,
the tires are low profile, perhaps this is the reason that I don't show the tires size you describe.
When I measure diameter while on the bus it comes out as a ..ooops maybe it is a 35" tire. I was thinking it was 30" either way it is not more than 35. I will have to look again.
But WHAT I DID. I measured diameter. set the dial to match up with my gear ratio and checked the rpm/mph and everything matched up with actual speeds to rpms. I then moved dial to a new gear ratio and it gave new rpms. So i know the calculations are correct.

current readings: 2400-2600 rpms is 50 -55 mph. Basically that varies based on grade and wind. I can sometimes run 58- 60 @ 2600 rpms. and I can usually run 55 @ 2400 rpm.

According to the engine sticker it is a 190 horsepower setting.

I have the AT 545. It is 100% there is no shifting issues, hard shifting, nothing completely smooth.

I have plenty of accelerator left to run at 55mph.

2700-2800rpms is the max range I got from my cousin who runs a regional trucking company when he looked up specks on the computer.

According to my manual, the power range torque values are listed under 2400 rpms.

It shifts around 2100-2200 rpms depending on gear and load.

I realize that there would be more load on the tranny. I hope not too much.

I don't have exact figures in front of me but I basically recal that 66mph + would be the same rpms as 55 is now and I could run 55mph at somewhere around 2100-2200 rpms, which should give me considerable fuel mileage over a long trip and less engine wear.

I am glad to know that everyone is sure the engine has the power for the change.

You are right on the tires. I looked into 11R and I was told by the tire guy that I have plenty of height but the axle will not maintain the minimum spacing between the duals. I am not certain he is right. he was in a hurry.
Originally i was going to change the tires only to get just enough change and then later the gearing to get really good ratios but after the tire guy I decided that the nice turning radius with this tires was a plus and the gear change was enough.

My plan is to look at your calulator and to check into the extra load upon the tranny if I gear change.

Thanks again and any more talk is welcome

[JoeA]

Ok the online calculator says 4:11 gives 63.3mph with a 35" tire which is probably right.

It shows I am at 54.6mph @ 2500 rpm which is about what I experience.

I love the website for gearing .

thanks soooo much

[Les Lampman]

Hey Joe,

Welcome back.

Good luck on whatever changes you go for; it's a bit scary since we're not dealing with cheap components (even the tires!) but if you're comfortable with the numbers you're getting there shouldn't be any problem.

[eggman]

I'll add my $.02 into the mix........I have a 95 DT466 with the AT545 tranny and 4.44 gears. Running the 11R22.5 tires at pedal to the floor, I'll get max top speed of 73 mph. Checked with a car running behind me. Mileage is about 6-7. Keeping the land yacht at 65, pulling 7K pound trailer, I'll get right at 1 mpg with it. Also, with out the trailer, it still gets 11 mpg going down the road.