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Growler Enthusiast

Growler

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Newbie with questions...

 

I'm looking to go to a larger diameter tire, but since I still want to be able to install chains if I need to, I think that I need a lift to provide adequate wheel well clearance. I'm looking at a 255/85R16 so I can still use the stock aluminum rims.

 

Question 1: Anybody have any experience using chains with larger tires?

 

I understand that GM will only calibrate speedo/transmission for 265's. But since I have 4.10's, I'm thinking that as far as the speedo and tranny go, if I had my computer changed to reflect 3.73's (i.e. lied to it) and then went to a 255/85R16 it would basically be a "wash" (to within about 1.3%).

 

Question 2: Is there something I'm missing, or would this work?

 

Question 3: Would a dealer be willing to do this?

 

I want a suspension rather than a body lift, and the only ones I've seen are 6" (a bit higher than I really wanted to go). Which brings me to...

 

Question 4: Will I have any problems towing with 33" tires and a 6" lift?

 

Question 5: Will it effect the overall driveability of my truck?

 

And, although it's a bit of a topic shift (but related to the computer programming change question), I had a question about one of my "peeves."

 

Question 6: How hard is it to find a dealer who will eliminate or raise the 98 MPH "speed limit" built into the computer?

 

Any help appreciated.

 

-Growler

MiSdIrEcTeD_1 Enthusiast

MiSdIrEcTeD_1

Posted
Posted
No experience with chains, but I do know that a dealer is definately not going to eliminate the top speed limiter... Invest in a Hypertech... You can calibrate your speedo and all by telling it what size tires/ gears you have, as well as eliminate the limiter...  :fume:
JimWilson Enthusiast

JimWilson

Posted
Posted

Welcome to the forum...

 

Q1: Nope, never used chains and haven't seen anyone who has. As a basic rule of physics... if the number of revolutions per mile for a "different" tire size stays the same as the factory's original tire size (regardless of width) then the speedo will be accurate. But even if you go up 1 tire size -- and incur a 1 or 2 % difference -- the change is so minimal as to not really be worth hassling over.

 

Q2: ?

 

Q3: All dealers are different -- the one I bought my truck from has a horrible service department -- but yours might not have a problem with it. Of course, they'll most likely want to charge you $$ for anything they do.

 

Q4: It depends on what you're towing, but generally speaking it's not a good idea to tow anything heavy with a lifted truck. Also, larger tires are going to reduce the affective rearend gear ratio, which will sap a little power from your truck. That can make towing more difficult as well.

 

Q5: Lifts, and bigger tires (along with the new shocks you'll need), can definitely alter the ride and handling of your truck. But that's a matter of personal preference; some people enjoy the changes that a lift can bring, while others don't like it.

 

Q6: Forget it -- no dealer is going to do that. Too many sue-happy people nowadays. :fume:  But, as previously mentioned, there are some companies who make performance chips and tuners that allow you to remove the limiter and tune for different size tires. And those can make a noticeable performance increase.

Growler Enthusiast

Growler

Posted
  • Author
Posted

Thanks for the replies. A programmer may be in the future to get rid of the stupid top speed limit (lousy rotten lawyers!).

 

Also, I noticed in a recent post over on TheDieselPage that Rancho apparently has a 4" lift kit out for GM trucks, which is probably more to my liking for driveability than 6".  I really only need about a 2-1/2" lift.  I like how it drives now and really just want to get bigger meats and still be able to chain up - some of the places I go require it at times.  And after looking at some more pictures, I think 285's would look better than 255's, so I may go ahead and spring for new wheels.

 

I guess I need to learn which programmers do what (for an 8.1L/Allison) to figure out if it's worth getting one.  But Hypertech's web site doesn't list a 2003 GM version (yet), and according to their site the 2002 8.1L/Allison version won't change shift points, shift firmness, top speed, or even correct for tires/gears. I thought (read here somewhere) that no aftermarket programmers could change the Allison shift-point programming? And going from 245's (30.5" dia.) to 285's (32.8" dia.) is a 7.5% change, i.e. if the engine had been running at 2500 RPM in a given gear at a given speed, it would now run at 2325 RPM in the same gear at the same true speed (and read about 60 mph when I'm doing 65).  The Odo and the fuel economy calculator on the DIC would also be off by 7.5%.

 

But I happened to notice that the ratio between 4.10's and 3.73's is about the same as the ratio in diameters between 285's and 245's.  So if a truck came with 3.73's and 245's, and then the rear end and tires were swapped to 4.10's and 285's, things like shift points should be essentially the same as before the swap (same RPM AND Speed). And the speedo shouldn't even need recalibration. :cheers:  

 

Working backwards (since I already have 4.10's), if I just told the dealer that I had had 3.73's installed and wanted the Allison and PCM reprogrammed for 3.73's, then drove straight from the dealership to the tire place and had 285's installed, the drivetrain ought to behave about the same as if I really had factory 3.73's and 245's. Right?

 

Anyway, I thought it was a pretty cool idea if it would really work. I suppose what I'd be giving up is some low-end torque.... Oh well, TANSTAAFL.

 

Power and torque-wise I think I'd still be OK for towing, as I mostly just want to tow a boat, and most people who buy an 8.1 have 3.73's and apparently to do all right. Even a pretty big boat is pretty light compared with what I see some of you guys towing!  Height may be more of an issue here.

 

Things would sure be a lot easier if GM had just designed our trucks for larger tires!

JimWilson Enthusiast

JimWilson

Posted
Posted

The speed limiter is not only because of the lawyers -- although I'm sure that plays a big part -- it's because you're driving a large truck without high-speed rated tires. Be honest here; what the h#ll does anyone with a 6000 lb truck need to be doing drive over 100MPH??  :cheers:

 

I'm sure 4" lifts are available, because 6" lifts are a bit much for most people. I'm just surprised you only found one being offered.

 

I don't think anyone has the ability to reprogram the Allison yet -- I don't think anyone's "cracked the code" so to speak...

 

Depending upon what you're towing you should be okay. The 8.1 is a beast, even if you only have the 3.73's in the rear, so unless it's a yacht you should be okay.  :thumb:

MiSdIrEcTeD_1 Enthusiast

MiSdIrEcTeD_1

Posted
Posted
The speed limiter is not only because of the lawyers -- although I'm sure that plays a big part -- it's because you're driving a large truck without high-speed rated tires. Be honest here; what the h#ll does anyone with a 6000 lb truck need to be doing drive over 100MPH??  :smash:

Thats exactly right... And also, Im not sure about all states, but here in NM anything over 100mph requires a felony stop! That means guns drawn, lying face down, the whole bit!! A few of my friends have had the unfortunate experience of it...

:eek:  :cheers:  :flag:  :eek:  :eek:  :nono:  :nono:  :nono:  :nono:  :nono:

Growler Enthusiast

Growler

Posted
  • Author
Posted

Well, one nice thing about going over 100 mph - it gets you out of New Mexico sooner!  :smash:

 

But seriously, it's not that hard to hit the speed limiter when passing, which is when I DON'T care to have some darned computer telling me I can't have a little more acceleration.

 

On the other hand, I hadn't realized that the SteelTex weren't speed rated.  I don't mind having a heavy vehicle going that fast, but I'll think harder about it next time with this knowledge.

 

The last time I ran into a "speed limiter" was going through the desert in a 2002 F**D Crown Vic that I had rented.  As I recall, it was set at almost 120 mph.  :thumb:

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