Gear Ratio - Speed & Mileage Sweet Spot? - And other Stuff

[jhagan]

Not sure what you're referencing.. I've never had Allison deny unlocking 6th and I've had it done to several buses. Allison entertain the job unless you have an OEM letter stating the bus can be equipped.

[Bon Voyage]

For my md3060 6th gear unlock I signed a waiver then got the permission letter from Thomas. Then signed another waiver for Allison and they reprogrammed for the 6th gear. The 6th speed in that tranny only makes a small difference (0.1) but any bit helps. RPM goes from 2500 in 5th to 2250 in 6th at 65mph... I think. Close to that anyway...

My old 2002 Thomas got about 6mpg with 5 spd md3060 cummins 5.9 and 6.1:1 rear diff and governed at 65 mph. My newer 2001 Thomas gets about 8 mpg with 6 speed Cummins 8.3 md3060, 5.4:1 rear diff and a 14” roof raise (less aerodynamic) no govern.
Both full size transit buses.

[jhagan]

The higher the speed, the greater the difference will be. A lower speeds, your difference will be around 250-300 rpm.

[shortbusak]

Has anyone changed the year ratio of a single gear in an Allison 2000? I have a CAT 3126 and Allison 2000 that drives great on the flat land but is unable to maintain rpm’s in first gear when climbing the hill to my house while towing my boat. Not sure of the rear end ratio but I’m happy with highway speeds (65 mph at 2220 rpm). The boat is pretty light but I would really like to tow more one day. Been wondering if it’s possible to increase the ratio on first gear so that I can really haul up hill. Or maybe I just need more HP. Anyone have experience with tuning up a 3126?

[jhagan]

What's your rear ratio? Also, what is your HP?

[Brad_SwiftFur]

Originally Posted by jhagan

5) In reference to ratios - the indicated ratio is the number of times the drive shaft rotates per one rotation of the wheel. While a lower ratio is more fuel economical, the trade off is torque. You may have a slower bus if your ratio is smaller.

Close. A numerically lower ratio (mathematically) translates to a faster top speed. You are right about torque, on a low spec engine with a low HP rating, it may not have enough torque to maintain a given speed using a "faster" gear ratio (all other factors being unchanged), since the gears do "multiply" the torque.

[shortbusak]

Originally Posted by jhagan

What's your rear ratio? Also, what is your HP?

Ratio is 5.13 and HP is 175. There’s a lot of room to grow on the power but I’m new to diesels so any advice on the best way to do that is appreciated. We could take that to a separate thread though because I’m sure it will take several years. I want to instrument the hell out of the thing too.
.
I am curious to know about swapping the actual gears in the Allison 2000 though. Is that an option or is power The only way to improve towing up hill and still have the ability to run 60 on the hwy? The commute to my house is 2000 vertical at 8-9 deg in some parts.

[Bon Voyage]

I’ve wondered about switching out a transmission gear to get better rpm’s on the highway but you’d have to pull the transmission out and open it up which isn’t cheap. If the money isn’t an issue you’d probably have to talk to a transmission shop to get a better answer if it could be done.

[Booyah45828]

I've never heard of anyone swapping gear sets in an allison. If you're thinking the gear ratio is the issue, the allison 2500 has a 3.51:1 first gear ratio vs 3.10:1 that is in most.

But in 1st gear, you'll have an overall ratio of 15.90. Couple that with the torque converter multiplication and you should be able to get up the hill without problems.

I personally think you have a power issue if you're struggling to get up a hill like that.

[wrenchtech]

This is a pretty sweet spot on the 2008 Thomas Saf-T-Liner HDX that I’m selling.

[FamilyCircus]

I lucked out on my bus, it dropped in my lap before I had any idea what to look for.

Its a 2000 Amtran 30ft Flat nose with a dt466e 215hp/620tq
Transmission is MD3060 5 speed
Rear End is Eaton/Dana N190 with 4.44 Ring and Pinion
tires are 11R22.5

I cruise 65 at about 1700 rpm, it will do over 75 but why would you need that much speed, tires are only rated for 75mph.

Im going to get the 6th unlocked one day, then retune for 230hp/660tq because it has a flatter torque band from 1400 to 1900 rpm right where 70mph cruising would be.