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Tires/Gears/Speedometer And ABS

G

gandolphxx

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Jan 24, 2002
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Houston, TX
For those of you who are changing to tires [or gear ratios] that are not stock. When you do this you change more than you think, you change the shift points and the ABS handling in addition to increasing the inaccuracy of the speedometer.

Options to correct this situation:

1. Your dealer can adjust if they have a size [OD] equivalent, and of course they can adjust between 3.73 and 4.10.

2. Hypertech HPP3 can do most of the work, but has limited settings - a tad coarse.

3. Superlift TruSpeed has a relatively new device that will allow for fine tuning of the OD and its relationship to the PCM - wires inline. Reveiw of the TruSpeed - this doesn't appear to do gear ratios [?] - have the dealer adjust and then fine tune the tire size

4. SCU2000 - seems to have become obsolete, had one bad experience with it [DOA] and doesn't appear on the Jet webpage anymore. This also did gear ratios. ;D
 
G-xx-

Such is why Town Fair Tire ( A local chain with good prices) would not even touch my Av with anything other than keeping the same OD (sure you can go with a larger rim, but the rubber still needs to equate to stock OD . . .) Since my Av came with the 16 inch wheels/rubber with an OD of roughly 30.5, going to an OD of roughly 32.5 is something they would not even touch. Loosers - their lawyers had scripted them pretty well. I guess they turn their back on any customers with creative tire choices. After doing the rim&tire upgrade myself, I did notice some weirness witht the ABS, but I haven't seen it since correcting my OD with the Hypertech PP3.

As for the ABS issue, Town Fair Tire mentioned that too, but I am not sure how it plays out vis-a-vis speedo error.

ps. you forget to mention the affect on the speed-sensitive volume on our radios, no doubt impacted as well ;D
 
My Av (for some odd reason) had the 16 inch (non-Z71) wheels on it (and I got an appropriate price break) but then when I went to have a GM tech friend of mine fix the computer for my new tires, it was programmed for the 17" wheels that were supposed to be on the Z71. Go figure.

Anyway, after I put the big meats on (33.1" OD) I had him tweak it, but the list of options in the programmer is quite limited and the closest I could get was something like 32.0.

Now, when I first put the big tires on, it felt a little strange, but I think it may have been just getting used to them. Tried some "panic stops" and didn't notice anything wrong with the breaking. Stepping up to the largest tires size (i.e. closest to my tires) seem to fix the transmission shift points (they felt a bit wierd initially too).

I'm guessing the speedometer is still a few mph off, (worse at higher speeds of course) but the transmission and ABS seem to be satisfied with the largest computer selection, even though that's not as big as the tires.

If anyone comes upon an inexpensive way to further remedy the situation, please let us know! :)
 
Gandolph had me so freaked out :eek:

that I called the outfit that did my lift and tires and asked them if they reprogrammed the UPD for me.

They let me know that they did but they could only do it to 32" tires, so I am in the same boat being 3.9% off.


 
flynhigh said:
Gandolph had me so freaked out ? :eek:

that I called the outfit that did my lift and tires and asked them if they reprogrammed the PCM for me.

They let me know that they did but they could only do it to 32" tires, so I am in the same boat being 3.9% off.
Click to expand...
Glad I could help, didn't mean to panic anybody. :rolleyes:
 
Crexis said:
...after I put the big meats on (33.1" OD) I had him tweak it, but the list of options in the programmer is quite limited and the closest I could get was something like 32.0.
Click to expand...

I think I must be doin the tire-math wrong. Either that or the actual size of tires is not what the name says it is. Lemme see if I have this right.

Crexis's tires 285/75R16

285 mm wide / 25.4 = 11.22" wide
11.22 * .75 aspect ratio = 8.415" section height (rim to edge).
8.415" section height x2 (both sides) + 16" wheel height = 32.83" overall tire height.

Where did ya get the 33.1" number from? Is that because their numbers don't correlate exactly? Or somethin you measured yourself? Or... something else I can't figure out maybe?
 
I got that number from BFG's manufacturer spec info. I also figured it out the way you did, but chose to use the BFG number to shamefully maximize the impact of my post ;)

http://www.tirerack.com/tires/Spec.jsp?make=BFGoodrich&model=Mud-Terrain+T/A+KM

Anyway, I suppose I'll never know exactly short of trying to get an accurage measurement myself. In any case, the ABS, tranny, and Speedo all seem to be working well enough to satisfy me. That doesn't explain what to do about a lifted Av though... I would think that going much larger than 33" would still cause problems even with the computer set to its maximum available size...

 
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