3.42 gears in a '05 2500 hd Dmax?

[skark_burmer]

I just bought a 2005 Ext cab short bed 4wd Dmax/allison and want to install taller gears, it came with 4.10. I called Precision Gear and was told that there were no options for my truck.
I dont use it for towing and mostly all the miles are highway so id like to lower the RPMs as much as possible. Any advise?

[lovette]

I thought all Allison tranny 2500/3500's came with 3.73's.

Anyway, I went from 245/75/16 stock tires to 285/75/16's. RPM's at 70mph (by GPS) dropped from 2200 to an even 2000.

[More Power]

All Duramax 2500HD/3500 trucks get 3.73 gears. The 8.1 HD trucks can get either 3.73 or 4.10's.

Contact Randy's Ring & Pinion (http://www.ringpinion.com) to see what they say.

Incidentally, these trucks should produce ~2000 rpm at 70 with factory LT245/75R16 tires. Lil Red (http://www.thedieselpage.com/features/project2-16.htm) runs at ~2000-rpm at 80-mph (with 31" tall tires and 3.42 gears). You wouldn't want that gearing in a truck that does any towing.

JIm

[Quack_Addict]

MP,

The SS wheel isn't forged - it's cast.

[6APPEAL]

More Power,
My completely stock 2005 CC D/A turns 1800 rpm at 60, 2000 rpm at 65, ~2200 at 70 and ~2400 at 75. There seems to be some discrepancy between the rpm/speed. Some folks are seeing the same numbers as me and other have the same numbers as you. Maybe no TC lockup? That is good for 200 rpm in my turbo Buicks. But I have no error codes for point to that. I would like to see a little less rpm at highway speed. I'm thinking of a taller tire, but with GM frowning on that, probable not until nearly out of warranty. Which leads to the question of why GM put a 245 on the 2500's and a 265 on the SRW 3500's?
Thanks,
John

[More Power]

6APPEAL,

I wish I knew the answer as to why GM insists on 245's for the 2500HD.

As far as changing tire size and that relationship to the indicated vehicle speed and engine rpm - it makes no difference.

Changing tire size will produce a difference between indicated speed and actual speed, but the relationship between the speedo and tach won't change. This is so, because the vehicle speed sensor and engine speed sensor are both upstream from the tires.

If you do the math for the gearing, 2000 rpm should equal approximately 70-mph when in OD/lockup.

Lil Red is running with a Predator program that corrects for tire size and the 3.42 gears. Doing the math produces approximately 80-mph at 2000-rpm.

Jim

[mbowerma]

6apeal

My stock 2004.5 2500HD gets the same numbers as yours.

[More Power]

You all might be interested to know that the new six-speed Allison essentially gives you 3.42 gears when running in sixth (1800-rpm at 70-mph).

Jim

[gritz]

One question about changing tire size. According to factory information at www.allisontransmission.com (http://www.allisontransmission.com) changing tire size might be detrimental to the transmission. Quote:

"Q. Speedometer / Tires - What effects will big tires have on transmission operation? A. The TCM calibration is based on factory tire size and rear axle ratio. Changing either of these parameters will have some adverse impact on transmission operation, including features like grade braking and shift stabilization. For instance, we have noted that when P285 tires are installed that the transmission generally does not shift to 5th range when towing heavy loads."

I wanted to go from 245/75/R16 to 265/75/R16 on my 2004 Duramax ... but now I'm wondering how that translates to "actual" experience versus factory warnings?

[DmaxMaverick]

Although I'd take Allison's word over GM's anytime, that statement is precautionary. There could be drivability issues with larger tires. I find those issues to be beneficial. I've been running 285's for about 65K. I tow heavy, near GCWR frequently. There are times when it doesn't want to shift into OD, but it will if I ask nice. Never an issue. Grade braking has always been excellent. Of course, with larger tires, the speeds up/down shifts occur are slightly higher. IMO, the truck seems to be happier with the larger tires. As a last note, my highway mileage increased, city mileage didn't change.

[ADub]

Same here, put on 315/70/17s and my city mileage didn't change, highway mileage increased. My towing mileage decreased a bit, but the truck has no issues going into 5th with a load. I haven't towed more than 6k with the 315s.

[SLT556]

Can't these thing be re-calibrated with a Tech-II scanner?

[DmaxMaverick]

Originally posted by SLT556:
Can't these thing be re-calibrated with a Tech-II scanner? No. The ability is there. GM refuses to allow it.

[mark45678]

Greg L. was working on upgradeing a 2004.5 lly truck to 2006 allison 6 speed spec! this was back about early 2005, I havent heard any more.... it maybe a better idea to have more gears then lower ratios ? cant say for sure.

[chipper]

I had a friend who owned a 2003 Monaco motor home, & he said it had the 8.1/ Allison with the 6 speed. It had been hooked up for 6 speeds with a Monaco conversion.Anyone else heard of this?

[rat4go]

New to the forum, but have some insite here. Bigger tires act like a heavier load on the entire truck. Brakes work harder, driveline works harder, etc. The 285/75R16's are good for about a 7% drop in RPM (compared to stock 245 tires), where going to a 3.42 from the 3.73 would give you about an 8% drop in RPM at the same real speed.

I've been running my '03 Dmax with 285's since day one. I haven't and probably won't max the truck out from a GVWR or GCWR perspective, and with the larger tires, I consider the max as about 7% below published numbers to be safe with the brakes and driveline, but with the 16000 GCWR I typically run when towing, I've never seen any adverse issues with the 285's other than the speedo reads lower than actual groundspeed by the same 7% or so. Since the vehicle doesn't have any way of measuring actual ground speed vs the speeds it can measure (tire RPM or transmission output shaft RPM, etc) the only thing that I can think of that may cause an interaction issue is if the computer has some fancy way of predicting vehicle weight to do grade braking based on weight. If so, it'd think the vehicle with big tires is heavier than it really is.

From a fuel economy perspective, you'd think that you'd get a bit worse for city (more engine torque required to get the same acceleration), but on the highway, you're not spending much time accelerating, so the lower rpm should help things a bit.

Lastly, a bit of a tongue in cheek note to DmaxMaverick. ;) I'd trust GM about the same as Allison. They're the same company - Allison is part of GM's Powertrain Division!

[DmaxMaverick]

Rat4go

Welcome to the forums!

I agree with you about the larger tires. I've been running with 285's for most of the truck's young life. Mileage...No loss in city, gain on highway.

The GM/Allison statement was directed toward service shops and the engineers, not the bean counters. Allison is a division of GM, but it is a division. They are not the same. If you want "real world" technician advice, visit an Allison dealer/service dept, not a GM car dealership. Some dealerships have competent Allison technicians, but many do not. Anyone can read/quote service manuals and bulletins.

[Heartbeat Hauler]

I received an e-mail from Gear Vendors (GV) yesterday and they are working on an aux. overdrive unit for the Allison 1000. GV said it would probably be in the $2995 price range. I don't know what an Allsion upgrade-to-6speed would cost, but it might be close. Anyways, the GV unit would give you a 10 speed with dbl overdrive.
JP

[rat4go]

DMaxMaverick,

OK, now I understand the context a bit better and I'd be inclined to agree with you. I've never been to an 'Allison Service shop' so no experience there, and I tend to do my own service, so in honesty, I haven't talked to the regular GMC dealer about my truck much, either. I'm a bit fortunate in that I have an indirect info source for technical (friend in the GM engineering ranks). He's not directly tied to the Diesel/Allison stuff, but he has contacts who are. Since it's sort of indirect and he's got to burn favors to get info for me, I try to use him only in emergencies so as not to burn out him and his contacts, but it sure is handy. :D