Hi,

I don't have a Gear/Tire Calc Tab as seen in this video:


My available tabs under Speedo are Calibration and Limiter. Not General, Limiter, and Gear/Tire Calc.
I have HPTuners 3.0
On the Calibration tab I have Final Drive and VSS Pulse per.
My Final Drive Ratio is showing 1.00 which I know isn't my actual final drive, and my VSS Pulse per shows 69,192 mi-1
How would I go about changing my tire size now?
The overall diameter of my new tire is 26.1"

Thanks,

 

Anyone? Seems like it should be a fairly basic question? No?

 

Just a rough adjustment based on the differences is VSS of 68847.36552 for your 26.5" tire vs the stock 25.97" (used the number from a discount tire calculator so unless someone has the actual diameter of the stock wheel and tire setup)

Verify with a gps speed reading over a longer flat road at higher speed

 

Sometimes you have to wait more than two hours to get a reply, especially in the morning

 

He didn't make it to two hours, he was just at one hour

 

Sorry, tires should be here any moment. Wanted to get this figured out sooner rather than later, as I have a long trip ahead of me.


My new tire is actually 26.1 not 26.5
Still not sure how to calculate this on my own though, or
why my final drive shows 1.00

Thanks,

Jay

 

final drive in every tune ive seen is 1.00

 

Why though? I don't get that. Our final drive isn't 1.00.

 

You're looking at an insignificant difference, only going a bit larger than stock

 

There are parts of every calibration that arent used for that specific application. In this case, they use vss pulse per mile instead of some other calculation.

 

Also I should note I did calc it for 26.1, not sure where I got the 26.5 in the first place. Just used a ratio then applied it to the VSS pulse number

 

so, how would I calculate VSS Pulse per for 26.1 ?

 

Wouldn't stock be more like 25.09? That's what discount tire showed me for a 225/40R18

My new ones are 26.1"

Not sure how to calculate what to use for the VSS though.

Could really use some help, and still have no idea why the 1.00

 

You just do a ratio, take 25.01/26.1 * 69192 = 66302.37

You don't need to calculate it out, just the difference so a larger diameter tire takes less rotations per mile, therefore the VSS sees less signal counts. At least thats my understanding of the system, I've never played with it, thats why I say check via gps signal

 

Thanks, went with 66302

Still curious about the final drive of 1.00. I mean my stock and all my tuned files all say that, so guessing it's OK that way, but I still don't get it. I know the final drive isn't 1.00, and I've actually replaced my final drive, so would have expected this to change one way or the other. I mean this is there, so I'd think it'd be used, but guess not, or???

 

You know how GM uses the same parts across multiple cars? Well, they do the same with ecu programming. There are features in the programming that do nothing even when adjusted because the ecu does not use it to run that specific car. It's easier to code to disable than code to remove or replace.

That's my understanding of it.

Test it out by changing the final drive value and see what happens.

 

 

here is the reasoning behind the final drive ratio being set at 1.00. it all has to do with the location of the reluctor wheel for the vss. in the cobalt, and most fwd vehicles, the reluctor wheels is on the differential carrier, the reluctor wheel turns one rotation for every rotation of the wheels. basicly the vss isnt effected by the actual gear ratio of the vehicle. in a rwd or 4wd application the vss reluctor wheel is in the transmission or transfer case, it turns whatever the gear ratio is times faster than the wheels. in this case, the pcm needs to know the actual gear ratio to properly calculate the vehicle speed.

 

Im working on calibrating my speed sensor as well. Only Difference is mine is off due to final drive. I have tried changing the ration to no avail. I need to mess with the VSS pulse per mile, but I dont know what the equation would be since my tire size is stock (Although a slight stretch, but from my research, that only changes section width, not overall diameter.) Any ideas on how to adjust the tune from a 3.82 to 4.45? Im about 6mph off @ 70mph verified by gps.
Thanks for any input.

 

bumb

 

Change the value, check, adjust again.

 

any idea on where to start? OR to even go up or down, the value is at something like 67,823. Im guessing ill just have to play with it? No real math involved?

 

Look at what ecaulk said earlier in this thread. Make a ratio and adjust from there

 

If you are reading too fast, adjust UP. Too slow, adjust DOWN. Just make a ratio of [IS]/[SHOULD BE] and you should be set. For example, 76/70=1.0857. Sharkey explained why the speedo shouldn't care what your final drive is anyways.

 

Just an FYI The Gear/Tire Wizard is under "Edit" in Ver 3. if thats what you are referring to in OP.