Anyone with tire size performance problems?

[TraceF]

I am a new Duramax owner with questions. I have a 2003 HD 2500 4x4 CC SB D/A with 1000 miles on it. I put BFG 285's on it after a little bit of plastic trimming. Towed for the first time yesterday and was very dissapointed. My boat is a 27' offshore fishing boat, needless to say pretty aerodynamic compared to some of the hoe's, tractors, and 5th's you guys are pulling. It scales at almost 10,000# with gas, water, and ice, trailer, etc. My 2002 K2500 4x4 454 would easily outrun the Max with the same load up to any, repeat any- speed. At times I had to keep the pedal to the floor to keep the rig going 60! Spent the day at the dealer. Dealer says tires are confusing the computer and costing power... they produced a GM Tech Bulletin # 01-03-10-010B (10-14-2002) which essentially says not to work on diagnosing power complaints if the oem tire size has been changed. It SPECIFICALLY says dealer and owner should not alter tire size. Any input on tire size related performance issues? Thanks

[TBC]

There is a Allison web site that discusses the tire size issue and the trans programing. Essentially it says changing tire sizes afects programming and performance. There are also many posts on this website that discuss tires and the affects and no affects and it is very confusing. I would have to consider seriously what Allison says before I change to tires as large as 285's.

Tom

[NeverForget]

see my reply in the dmax section.

[Mustfish]

TraceF,
I feel your pain and frustration. I did the same thing and got the very same story from my dealer. I was using 265's and had to push the pedal slam to the floor to get a 4k lb. boat up to 65mph. It also took about 2 miles of interstate to do it. The Allison web site warns against tire size increases. I put 245's back on my truck. I still think my 98 Ram 360 can out pull it but I sent that down the road so who knows.

[TraceF]

Well- at least it's not just me. This seems odd though. I see these trucks lifted with tires a lot larger than this running around. The general consensus seems to be to let the truck break in and let the management systems get used to all the related conditions. I'm going to wait and see if it gets any better. Meanwhile I am exploring other threads considering power enhancement modifications. Superlift has a VSS module I am going to look into tomorrow when I can get their tech staff on the phone. If it doesn't improve I'll sell it. I don't want it so much that I will live with design limitations that are too extreme. GM should have thought this out, people have been putting larger tires on trucks since the turn of the LAST century. With the wheel well sizes on these trucks you would expect they were going with the trend.

[CPMac632]

Can you put the stock tires back on. If you have them I would because I doubt that an inch of tire will make that much difference so I would look for something else. 285s on my truck make a slight difference in acceleration but no difference in holding speed with a trailer.

[RWTD]

To give notice, the DiabloSport Predator will correct for tire size and gear changes!

I will have more information to post soon!

[ 12-23-2002: Message edited by: RunninWithTheDevil.com ]</p>

[svpdiesel]

I spoke to Diablo on 12-20, they said Predator for Duramax not available till after Jan 1. Also said it will NOT address anything in the transmission, because the tranny has a separate brain. Can add anything to the relatively small amount of info available? Plenty of interest on this forum...
S

[RWTD]

The transmission does have a separate brain, and eventually we'll be able to tap into it properly, but at this time the Predator will not have this capability. I will keep everyone up to date on the ability to adjust the transmission functions in the future, though.

I am a very close confidant of all the guys at DiabloSport and you can be assured that any and all information I give you comes straight from the horses mouth (as well as my own experience)! I am on the phone w/ them multiple times daily, email, as well as AIM (inst. mess.). I am one of their largest dealers.

I have the most experience w/ the Predator of any person, as well as more R&D than any other, so you can be rest assured at what information and knowledge that I bring to the table will consist of only the most important tech available.

Feel free to ask away!

P.s. The unit will be available on or about Jan. 1st. The holidays may slow its release down slightly.

Very Sincerely...

[ 12-23-2002: Message edited by: RunninWithTheDevil.com ]</p>

[Kennedy]

Transmission shift points are based on OSS (output shaft speed) in RPM. The ECM and TCM do not care what size tire you have, BUT the respective loads go up as you are basically trying to pull about 10% more gear which of course requires more pedal which...

I guess what I'm trying to say is that correcting for tire size will have little/no effect on anything besides the speedo/odo.

[mtomac]

I have 285x75x16 BFG All Terrain's on the stock rims. So far I've only towed 13,000# with these tires. I didn't notice much difference with the HOT juice on level 1&2 towing. I'll have to wait till this summer to see if a 16,000+# trailer with the larger tires will make a difference with the juice. Without the juice you might notice a power loss empty and would definately notice it towing.

I don't think the allison shifts any different with BFG 285's than it did with the stock 245's. I asked a mechanic at a dealer I trust about larger tires and the Allison. He told me the same thing John mentioned above, "Transmission shift points are based on output shaft speed and RPM. The computer doesn't care what size tire you have." The Allison still upshifts and down shifts at the same rpm's but the truck is just going a few mph faster than it was before. Just like when the speed limiter cuts off at 97 I'm actually going a little over 100.

Performance doesn't seem to be lacking running empty with the Hot Juice on level 3. I'll have to wait till the dragstrip opens to see if the bigger tires hurt my 1/4 mile times any.

[GMC-2002-Dmax]

285/75's on 16"x8" M&T rims.

Absolutely NO PROBLEMS, even towing 7000 lbs. plus.

Put Them ON!!!!!!

GMC

[Steve Joneli]

While I don't know anything about the computers in the newer trucks, I have to agree with John Kennedy about changing tire size and how it affects gear ratio and performance. A slight tire change can make a world of difference. Years ago I had a half ton 4x4 short box Dodge. I went from the stock tires to some really big mudder type tires. It turned the truck into a DOG. Using the gear ratio and tire size calculators I can see where my tire change basicly changed my gearing from 4.10 to about 3.00. Another example. A while back while filling my truck a Toyota 4x4 pulled in. Pretty nice truck. Had a Chevy small block in it and what must have been about 36" tires. I asked the owner about gearing and he said he had gone to 5.38s to compensate for the tires. Just my two cents worth on an otherwise boring morning.