Correcting speedo with vss pulses?

[griswold]

Does anyone know the procedure or proper way to do this. I have an 05 silverado 4x4 with the 4.8l and a 5 speed. When the tune is pulled from the truck, it shows an "error" in current tire size and the size calculator will not pull up. The truck has stock 3.42 gears and I went from a 265/70/17 to a 305/70/17. Any help would be greatly appreciated.


Jack

[Chris@HPTuners]

did your actual tire diameter change? all i see is you put wider tires on which shouldn't affect anything.

[griswold]

did your actual tire diameter change? all i see is you put wider tires on which shouldn't affect anything.

The overall diamater of the factory goodyears is 31.7". The tires I put on the truck (maxxis buckshot) have an overall diameter of 33.9". The calculated height of each tire is probably less but the speedo is off enough to get me in trouble. We were kinda stumped when it wouldn't let us change the tire size and calculate

[BLWNGTO]

did your actual tire diameter change? all i see is you put wider tires on which shouldn't affect anything.

Chris, no disrespect meant at all, but let me help you with the tire sizing. If you already know this, I apologize, just trying to help.

Lets start with the tire size that came stock on his truck.
265/70/17
What this means is as follows.
265= The tread is 265mm wide.
70= The sidewall is 70% of 265
17= wheel size
So, his sidewall is 185.5mm. Now, you multiply that number by 2 because you have sidewall both over, and under the wheel.
So, you have 371mm of tire. Now convert that to inches.
371/25.4=14.60 inches of tire.
Now add in the wheel diameter, and you have the overall height.
14.6 + 17 = 31.6 overall diameter.

The new tires put him at a 33.81" overall diameter.
Here is a nifty online calculator that is much easier than doing the math.
http://www.miata.net/garage/tirecalc.html

[Rammer]

In the Editor, Go to tools, unit conversion and there is a tire size calculator.

[BLWNGTO]

In the Editor, Go to tools, unit conversion and there is a tire size calculator.

Ok, I feel pretty dumb right now.
Sorry for the waste of page space.

[EC_Tune]

Actually, doing a roll out will give you the most exact tire height as the tire "squishes" (for lack of a better term) a little reducing the rolling radius.
HTH
EC

[Chris@HPTuners]

Chris, no disrespect meant at all, but let me help you with the tire sizing. If you already know this, I apologize, just trying to help.

Lets start with the tire size that came stock on his truck.
265/70/17
What this means is as follows.
265= The tread is 265mm wide.
70= The sidewall is 70% of 265
17= wheel size
So, his sidewall is 185.5mm. Now, you multiply that number by 2 because you have sidewall both over, and under the wheel.
So, you have 371mm of tire. Now convert that to inches.
371/25.4=14.60 inches of tire.
Now add in the wheel diameter, and you have the overall height.
14.6 + 17 = 31.6 overall diameter.

The new tires put him at a 33.81" overall diameter.
Here is a nifty online calculator that is much easier than doing the math.
http://www.miata.net/garage/tirecalc.html

yeah, my bad, not enough caffeine before engaging brain into gear that day

[kindaslo]

so what si the solution when the stock file reads error? I have an 07 yukon I just put 26 inch wheels on, and can't recal the speedo, because the commit change button is ghosted.

[kindaslo]

has anyone solved this??

[EC_Tune]

You will have to manually calculate the tire turns per mile.

1. Do a roll out in inches.
2. 63360/roll out gives you tire turns per mile
3. Apply that value to the appropriate slots in the speedo file.
4. Take old tire turns/mile divided by new tire turns/mile & multiply your shift points & TCC lock values by that.

Remember as the tire diameter goes up the turns/mile go down and your shift speeds should go up with a larger tire.

[kindaslo]

You will have to manually calculate the tire turns per mile.

1. Do a roll out in inches.
2. 63360/roll out gives you tire turns per mile
3. Apply that value to the appropriate slots in the speedo file.
4. Take old tire turns/mile divided by new tire turns/mile & multiply your shift points & TCC lock values by that.

Remember as the tire diameter goes up the turns/mile go down and your shift speeds should go up with a larger tire.

is there a write up on the best way to do a "roll out"

[EC_Tune]

Mark the ground, rotate one revolution, mark the ground, measure the distance in inches.

[Kanati]

I know this is an older thread...but,

Your calculation is based on a rear diff gear of 1:1. You have to multiply your sum by the ratio of the rear gear (in my example...4.88) to get the trans revs/mile...right? There isn't an area in the speedo breakout for revs/mile.

Example...315/75R16 tire = 34.6 or 108 1/2 inch roll out or 34.6 * 3.14 (pi) = 108.644.

63360/108.644 = 583.19 * 4.88 = 2845.963 trans revs/mile

Doesn't this really all depend on the number of teeth on the VSS sensor?

[EC_Tune]

Yes that will give you VSS Pulses/mile

[Fast240Z]

I have just found this thread, lots of good info. I have been struggling with this adjustment for some time now. I was asked by a friend to correct his speedo. He has a 02' GMC Sierra 6.6 DSL and 36" tires. I did the rollout calc. (112.5") and div by 63360. I entered this number in the 'VSS tire rev per' field and then produced a ratio of change by dividing the old and new numbers. I then changed the 'VSS pulses per" field by this ratio. The result was an accuate speedo but a constant "C0238" DTC. The "VSS pulses per mile" I came up with my way was 83578. I just did the calc. from this post and came up with 84027. The questions I have are can this small difference fix the DTC problem and if not how do I proceed?
I have noticed in my 4L80E tune I have a feature which automaticly makes this calc. and is not available for the alison.

Mark

[EC_Tune]

Try this tire calculator.
I've put a couple hours into it so it should be right for most all tire size applications.

It's a ZIP file renamed to .txt so I could upload it.
Just save it to your drive and then rename it from .txt to .zip then unzip it.

Enjoy!

[Bill@HPTuners]

Sweet Doug!

[Fast240Z]

Try this tire calculator.
I've put a couple hours into it so it should be right for most all tire size applications.

It's an Excel file renamed to .txt so I could upload it.
Just save it to your drive and then rename it from .txt to .xls

Enjoy!

Very nice work. I entered all of my info and came up with the same numbers I am using already!? Can you tell me how this system works? If I put in the stock numbers the speedo is off but no DTC's. If I put in my numbers then the speedo is spot on but the DTC's return. I am not totally clear on how th two numbers relate to each other.

Mark

[EC_Tune]

What DTC's are you seeing? If it's ABS related and you have much bigger than stock tires and/or a gear swap it's simply the ABS system whining that the front and rear sensors are out by > 15%. A Tech 2 can fix that for you.

And did you enter 40 Pulses per Rev trans output? That's the number that most trucks have.

If you need to fudge the speedo you can put in the VSS Pulses per Mile that you calculated with the spread sheet.