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Re-sizing and speedometer

A

AV Captain

Full Member
Joined
Nov 12, 2003
Messages
28
Location
Seattle,WA
Does anyone know how to re-calibrate speedometer/odmeter
after changing wheels/tire circumference from 16" to 17"?

thanks Pete
 
You can use an aftermarket peformance programmer such as the Hypertech, Predator, or Superchips or if you have a good relationship with the Service Mgr. at your dealer they can do it for you, or if the difference between the 16" and 17" is minimal you don't have to worry about it.
 
Any calculations would be based on the tire diameter. The wheel size is not relevant to this.
For example there are two standard wheel sizes (16" and 17") on the Avalanche, but the tire diameter used on both is only 0.1" different...
The reason is that the stock tire on a 16" wheel is 75% aspect ration and the stock tire on the 17" wheel is a 70% aspect ratio.
The manufacturer should be able to give you the true diameter of the tire (it's usually on their website).
HTH.
BTW the programming in the Av only allows for tire diameter changes in steps of 0.5" so anything less than that you don't have to worry about...
 
I asked the service manager at the chevy dealer where I take my Av if he could change mine when I put on 285's (about 1" bigger than stock 265's). He told me that GM will only let them change it for one size above stock (275's). Liability thing I guess?

I'm sure their computers CAN do it, he just said he's not allowed. I have no way to verify this. Guess I'll be FORCED to buy a programmer >:D
 
RichUF said:
I asked the service manager at the chevy dealer where I take my Av if he could change mine when I put on 285's (about 1" bigger than stock 265's). He told me that GM will only let them change it for one size above stock (275's). Liability thing I guess?

I'm sure their computers CAN do it, he just said he's not allowed. I have no way to verify this. Guess I'll be FORCED to buy a programmer >:D
Click to expand...

hmm, I will have to re-check my dealer... thy said they can re-calibrate up to a 33" tire....

Grey
 
Tree_Hugger said:
BTW the programming in the Av only allows for tire diameter changes in steps of 0.5" so anything less than that you don't have to worry about...
Click to expand...

I believe that the Super Chips programmer allows .25" steps. I'd have to find my shoes and go out and check which I will be happy to do if it's really important to you. Just ask

I put on 285's and found the speedo to be about 3% off. Haven't recalibrated it yet
 
AV Captain said:
Does anyone know how to re-calibrate speedometer/odmeter
after changing wheels/tire circumference from 16" to 17"?

thanks Pete
Click to expand...

You may not have to!

The real question is - did the O.D. change (that is the outside diameter). If you had the stock tire size on the 16" your factory tire is a:

265/70R16 --

OK so we take the 16" and multiple that times 25.4 that will give us the millimeters. That is 406.4 millimeters

Then we take the 265 and multiply that times .7 and then times 2. The 265 is the millimeter width of the tire, the 70 represents the aspect ratio, that is the sidewall height is 70% of the tire's width - we multiply this times two as the tire goes all the way around - so when measuring the O.D. we measure the width of the tire twice.

That comes out to 371.00 millimeters

Then we add the two together and divide by 25.4 to get the inches again

(371 + 406.4) / 25.4 = 30.60"

On an HPP III, which uses 1/4" increments you would pick a tire O.D. of 30.5"

========================

Now, if you changed to the factory 17" tire you've got 265/70R17 tires.

The 371 MM doesn't change - but we've added another 25.4 MM on the rim from 16" to 17".

(371 + 406.4 + 25.4) / 25.4 = 31.60"

Again, using an HPP III you would adjust to 31.5" as this is the closest choice.

All of this math is subjective. Air pressure, sidewall height and the type of tread lug can impact your total O.D. As an example I have 265/70R17 tires on my Avalanche but because the sidewalls are very stiff and they have traction lugs the O.D. is just a hair under 32" (measured).

So the million dollar question is:

1) What was the original size

2) What did you replace it with

Based on that you can then determine if you should be concerned about the speedo or not. Also just as an FYI the HPP III I understand will also adjust shift points and performance when you adjust the O.D. - so it goes further than tweaking your speedo.
 
Chief,

Thanks for the trip down the high school memory lane with the math. I thought I'd forgoten about that 8 year span ;D I think I saw Rod Serling!

So the 285 17's work out to an unloaded dia. of 32.7". Gained an inch over my stock 265/70 17's. Maybe I'll go out to the shed and see what the diameter is when it's sitting on the ground. The ol string around the tire and measure the string.
 
IBOHUNT said:
Chief,

Thanks for the trip down the high school memory lane with the math. ...So the 285 17's work out to an unloaded dia. of 32.7". Gained an inch over my stock 265/70 17's. ...
Click to expand...
Or you could have just read my post where I just gave that info without all of the math :p ;)
when I put on 285's (about 1" bigger than stock 265's).
Click to expand...
 
OK so don't flame me but I found this size vs. speed calculator out on the web. The only thing is that it is apparently from a Dodge website:

http://www.dakota-truck.net/TIRECALC/tirecalc.html

Pretty cool for figuring out speedometer error.
 
How good is a Nav system for checking your true speed in relation to what the speedometer is reading? I have the issue tires with 17 wheels on my 03-Z71. At 55 mph on the speedometer the Nav. system says I am doing 52 MPH. At 70 MPH the Nav system says 68 MPH. Which is correct? I have a Kenwood 2100 Nav system. I know it says after you change tires you have to do a re-calibration which I did after rotation of tires. You would think that for all the money that the Nav system is correct.
TEXAN
 
If your Nav system is GPS based, then it should be very accurate (as in easily within 1 mph), better than 95% of the time. To find out, you can set your cruise control on the highway, and over a 10 minute period, the GPS should be DNO (Dead Nuts On), excepting any instaneous readings which are out way of line.

My GPS reads down to .1 MPH. On 1 trip with the cruise on, it indicated I was doing 70.x most of the time, except for 2 disparate data points showed 34 mph and 209 mph.

Hey, you've got to admit, 20 mpg at 209 mph is pretty darn good ! :)->)

AVnAZ
 
ANnAZ I have the Kenwood 2100 GPS system and most of the time runs on 6 to 7 Sat and for almost $3000 it had better be correct. If I understand you right if I and doing a steady 55 MPH amd the Nav is reading 52 then my true speed is the 52 MPH and not the 55 on the speedometer. The same would hold true if the speedometer is a steady 70 MPH and Nav say 68 MPH. Is it normal for most av/trucks/cars to be off that much?

TEXAN
 
A cooperative GM service department can indeed recalibrate you speedo/ABS system to up to a 33" daimeter. Anything larger than that starts setting ASB codes. They would probably charge you 1 to 1.5 hrs labor for that. At $80.00 per hour you would be better off buying a programmer. Another thought, if you decide to have the dealer do it and then bought a programmer, you would have to have the dealer put it back to the way it was because the programmer records the base line from your vehicle and then gives you options from there. If if is already alterered you are going to modify already modified parameters. Not good. Usually ends up setting all kinds of codes, causing trany shift problems and such. Go for the programmer.
 
www.rx7turboturbo.com/robrobinette/tire_diameter.htm

Here is a cool tire diameter program just key in tire info and it give diameter.
 
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