Bigger Tire Computer Update

[Ranger354]

I purchased some 285's for my truck a couple of weeks ago. They handle nice and I have had no problems with them. Before I did this I got out my trusty GPS and checked the speedometer vs the GPS. They were within a couple tenths. Not bad I thought.

After the bigger tires were installed I made a few long trips and noticed the milage difference and checked the speedo against the GPS again. Alomst 4.5 miles off. The truck shows per the GPS to be going 4.5 miles faster then the speedometer. I guess I can live with that.

But wait, the service guy at my local chevy place said that they could recalibrate the computer and tell it the larger tire size thus getting a more accurate reading for speed. He aslo stated that everything (ABS, trany shift points, etc) gets its info from the speedometer.

It cost me $32 and the GPS tells me I am still traveling 4.5 mph fast then the speedometer.

What the heck is up with that????? You got clues?

[Rebel_Horseman]

Here's another good one. My room mate waanted to get his truck reprogramed for his 305's. Went to a dealer who is a good friend of his and the dealer told him that they could reprogram it but that it WOULD VOID HIS WARRANTY!!!!! :mad: My advice was to find another dealer. I think he's waiting on the Predator updates so he can do it all in one.

Reb [><]

[Black Dog]

It has been well documented on this site that the dealer can use the Tech 2 to reprogram the tire size paramenter for the ABS system only. Changing that parameter will not correct the speedometer, or affect the operation of the tranny controller.

[sdaver]

8 percent with a 285 or 305 thats 4 mph @ 50 or 8mph @ 100 :D dave

[ 03-07-2003: Message edited by: sdaver ]</p>

[SwampMonster]

I just installed 285's on my truck a week ago and I noticed the same speed differentials mentioned here. I also noticed that I am getting the same gas mileage. I travel the same route everyday and I know where and when I need to fill up, and I have gotten the same mileage with both size tires (245 and 285).


Lata

[LA DMAX]

Swampmonster,

Did you take into consideration the odometer being off and calculate it to be the same? If you did, that's good news. I thought that putting 285 would decrease fuel milage(given that your driving parameters didn't change). I ask because I was planning to change out to 265 or 285 but wasn't sure how much it would affect milage. Thanks

LA DMAX

[Ranger354]

"He also stated that everything (ABS, trany shift points, etc) gets its info from the speedometer."

What part of that did you not read Black Dog?

I guess the service guy doesn't know his &^%$! Even though he has been with them for 20 years. Or maybe they are just trying to make 32 dollars off of us the easy way.

What tells the computer how fast the truck is moving? Does it have another counter located on it? Or does it rely on the speedometer?

So, if it relies on the speedometer it would figure that the computer knowing the correct tires size would benifit this operation. Unless, the chevy place can only reprogram for 265s. You guys make the call.

[FirstDiesel]

Ranger354

I think Black Dog clearly understood what you wrote. I don't think you understood what he wrote. Try doing a search on this subject and you will see it has been discussed before.

Yes, you paid the Chevy guy and no he didnt't do what he claimed he could do. The results are he programmed your ABS but didn't correct the speedo or anything else.

Bottom line is you paid 32 bucks and didn't get what you were promised.

[blakmax]

I thought i was cuttin up the hog when i bought gm sps progamming for my pathfinder scanner.You can reprogram gm speedometers, but only what GM offered for tires when the vehicle was produced.At least thats what tech support for the software told me, after i bought it.

[Ranger354]

Guys,

I know it has been discussed before on the board, but I am too damn lazy to look for it. that I I guess I really don't care. Just telling you all what happend to me and asking if this happened to anyone else.

[SwampMonster]

LD Max.....I was not going by the odometer. I travel the same route everyday, and most days I do not make any additional excursions so I know the exact distances. Before I changed tires I knew exactly how far I could go (geographically not mileage wise) before the next fill up and after the tire swap I was able to make it the exact set of waypoints, and the amount of fuel to fill up was the same. Needless to say I was impressed.


Lata

[Comet]

When the ABS is reprogrammed in the controller for tire size, it is using a wheel speed sensor to determine if the tires are rotating.

Therefore, the parameters that are controlled by the speedometer (or its speed sensor) are independent from the ABS system. This is why a change to the tire size parameter for ABS doesn't affect the speedometer.

[afp]

The GPS is only accurate when recieving the proper sattelite signals and when travelling in a straight line on a level surface. Though it's precision in establishing position is good, I'm not sure it's accurate enough to reliable differentiate a 4 mph change on a vehicle travelling on the ground. The most precise GPS instrument approach mode available for aircraft--you need RAIM and WAAS--only guarantees a 3 meter CEP. While a ground GPS unit may have RAIM, WAAS requires a special ground signal and is/will be installed at certain airports.

Blaine

[LA DMAX]

Thanks Swamp, that is impressive if you're getting the same milage with bigger tires.
Laters

LA DMAX

[Ranger354]

Hey, It all sounds good to me. Guess If I can pay 40 grand for a truck I reallly shouldn't give a damn what kinda milage it gets.

So you really don't get the speedo callibrated at all. They are just blowing smoke up my a$$.