Tire Size And Speedometer...

[Carnac]

I traded the 245/75-16 Firestone original tires on my Avalanche 2500 for a set of 265/75-16 Goodrich T/A KOs (very nice tire, looks much better also).

The Goodrich tires are about 1.5 inches taller that the originals. I looked both sizes up on the TireRack tire calculator, the new Goodrich tires are about 4% larger diameter, so my speedometer should be showing about 4% below actual speed.

I checked my speed with my handheld GPS and found about 1 mph slower than actual at 45 mph. So, I am not too concerned about the calibration (not to mention, the actual odometer reading will be about 4% lower - a good thing).

But, my question is; how do you calibrate an Avalanche's speedometer? Is it a computer setting or mechanical (speedometer drive gear swap-out)? If it is computer calibrated, can it be set to be correct for any tire size (within reason)?

Just thought I'd see what other owner might have found out. Cheers, Jim

 

[gandolphxx]

Your dealer can reset for the 265's - they are offered - he can doit with his interface - typically 1/2 hour labor charge - 2 minutes - still cheapest way.

 

[Mikro Brew]

look at the date on this thread....I actually used the search..(and stopped when I found this post to add my .02 problem)

upsized to 285/75/16  from the stock 265/70/16...like a good boy I took the av to chevy dealer to recalibrate the speedometer.  took and hour..90 bucks...on the drive home the odometer registered 5.5 miles...funny that was the same as it registered to the dealers...broke out my gps, and cruised the highway.  60 = 64...funny that is what it was reading before the dealer...and what the tire calculator size thingy said the difference would be....so, back to the dealer...he rechecked and said it is programmed proper for his specs...and nothing he could do, maybe my gps is wrong and I got my odom numbers confused  (not sure how you confuse 5.5)..so I am done with this dealer.....I have read a couple of posts that the dealer cannot actually program for the 285/75 tire.....what are my options

 

[FLINTaVa]

I don't think most dealer service depts understand recalibration. The tire calculator charts aren't perfect, different brands may be slightly different diameter. Then add different rims in the equation and it mucks it up further. Some tires might recommend a 16-8 rim but people mount them on 16-7 or 16-7.5 rims all the time.

Speed/Odometers aren't perfect to start with, I wouldn't bother unless it's way off

 

[Mikro Brew]

my apples and oranges question:  will the speedometer being off effect the ABS ??

 

[sennister]

my apples and oranges question:  will the speedometer being off effect the ABS ??

Logically I don't see how it would. The wheel speed sensors don't really care about speed so much but a difference in speed between the four tires. If one rotates a lot slower than the other four then it sould kick in. If your spedometer is off by 4% or what ever as long as all the tires are the same size the truck should see that all 4 tires are rotating at roughly the same speed. If it was the true speed or not. That said I do recall reading some posts about the ABS light comming on after a mod but I think that was when they changed the gear ratios in the truck. That doesn't really make sense but I think I read something about that in the past.

 

[sennister]

Actually the one safety system that I could see new tires having an affect on would be the airbag system. The odds are pretty low that you would get into an accident and have this happen but I guess it could. From what I recall the "Next Generation Airbags" that began to comem out in 98 so all AVs should have a system like this look at the speed of the vehicle to determin how hard to pop or not to pop at all. So if you were boarderline with no, low or high deployment rates you may not get the right deployment for your actual speed. I would think the odds are pretty low that this would happen but I guess it could.