6.6 performance issues with tire size changes

[TraceF]

Early in the DMax days the GM Tech 2 was able to adjust the computer for tire size change. I have heard this has been done and turbo confirmed first and then a local dealer I spoke with confirmed later.

Since then, GM has removed this program option for DMax equipped trucks and issued a tech bulletin that essentially says not to try to diagnose driveability related issues on DMax trucks with non-oem tire sizes.

The only reasonable sounding explanation I have been able to get is that changing that particular parameter in the computer caused other driveability related issues.

I know there are dealer technicians as posting members as well as some other very knowledgeable people reading these forums.

1) Does anyone understand specifically why the Tech 2 programmer can no longer make this adjustment?

2) Will using a VSS module like the one offered by Superlift cause the problems associated with the Tech 2 changing the tire size info going to the computer?

[Black Dog]

From the very beginning on this board, it has been reported that the Tech II can be used to reprogram the ABS parameter for tire size, but that this does not correct the speedometer. I think this has always been the case with Dmax trucks, and it has never changed. Whether you can convince a particular dealer to reprogam anything is another matter altogether.

The bulletin that GM put out tells dealers not to try to diagnose transmission shifting problems unless the truck is wearing stock size tires. I can't see how they are not justified in taking that position.

[Kennedy]

I don't think the files were ever released or available. In order for them to become available, I am sure GM would need to get EPA cert. AND offer the optional tire/gearing to the public.

All the Tech 2 can adjust is tire size for ABS..

[TraceF]

Black Dog- I think the 10/02 bulletin I refer to isn't a tranny bulletin. When I get home I can give the bulletin number.

JK- what is the benefit of adjusting the ABS for tire size?

Do you think the VSS module will work OK and have no other effect on the computer systems?

[TraceF]

Black Dog-

The GM Tech bulletin is # 01-03-10-010B- (10-14-2002).

The header is:

Info- Inspection of Tire and Wheel Size Prior to Diagnosis for Transmission Shifts, Poor Performance, Speedometer, Cruise Control Concerns

It says "The ECM and TCM use RPM and VSS inputs as operational parameters for engine torque, transmission shifting, cruise control operation and speedometer indicated vehicle speed."

What I'm asking is will a VSS module give any performance benefit other than correcting the speedometer? The RPM input will still be lower but the VSS input will be corrected.

[Toddster]

What the hell is the issue with tire size and speedo correction ? Benefit, bigger tires = less milage on the clock....You know in your head your maybe 3mph off...Elementry school education tells you to back off a few mph with bigger tires ! Leave the damn speedo alone ! There, I feel better now...... :D Geesh, I thought I was anal ! ;)

[TraceF]

Todd- It's clear GM don't want the tire size changed, but... they provide for the ability to change the ABS for different tire sizes. This means something, I'm not sure what.

I understand less miles on the clock and this is a good thing.

What I'm getting at is IF the truck THINKS it has 245's on it will it perform better?

I guess the answer is to buy one and let my G-Tech tell me. This is the kind of experiment I was trying to avoid. I'd like to know the answer in advance of the $200 purchase.

[Toddster]

Just bustin' yer chops Trace old pal....I hear the stock Dmax is a real pig with 33 inch tires installed, don't know, don't care, never been there....With a 90hp, 125hp, or even 60hp box it just laughs at 33's ! With the hp I'm running it would laugh at 44's !

[TraceF]

I know where you're comin from. Try to send me the pics again.

[wild bore]

Maybe this will help, I tried to call them today. Guess they are not open on sat.

specmo.com click on calibration

[TraceF]

wildbore-

This is way interesting. Thanks !!!

I thought Superlift was the only game in town.

[TraceF]

I have been searching and reading the posts on this and today I took my own GPS out on the highway. When my speedo says 60 the GPS said 64.4 mph. This works out to be about 7% off.

I'm gonna call specmo tomorrow and see what they say. It doesn't seem like much but I'm still curious.