[squish72]

Im getting tires put on my car this weekend and I will need to recalibrate the speedometer. When looking at my tune I saw the fscm has a separate final drive gear ratio listed then the ecm and tcm have. When using the gear/tire wizard in hptuners it asks for the gear ratio, what ever ratio put in the wizard only changes the fscm.

Question 1. Why does my tune have two different ratios listed? (stock factory file was the same)

Question 2. When I adjust for my new tires should I use the ratio listed in the fscm or the actual 2.77?

[wnta1ss]

Since you are changing tires, then revs/mi is the parameter that correlates. Mine I've done in the ECM file's speedo section, never tried to do anything with the speedo in the fscm.

Noticed that with beta 4.3, now have parameters for tire circumference of the driven and undriven tires. My stock file says driven is 2150mm which correlates to the 748 revs/mile in the speedo. Wondering if setting these for new tires will make Stabilitrack happy? I've put short tires on just the rear before, set them in speedo revs/mi, and the computer was NOT happy, lights flashing and car felt like it did not want to go. Couple of years ago so these parameters were not there back then.

[squish72]

Originally Posted by wnta1ss

Since you are changing tires, then revs/mi is the parameter that correlates. Mine I've done in the ECM file's speedo section, never tried to do anything with the speedo in the fscm.

Noticed that with beta 4.3, now have parameters for tire circumference of the driven and undriven tires. My stock file says driven is 2150mm which correlates to the 748 revs/mile in the speedo. Wondering if setting these for new tires will make Stabilitrack happy? I've put short tires on just the rear before, set them in speedo revs/mi, and the computer was NOT happy, lights flashing and car felt like it did not want to go. Couple of years ago so these parameters were not there back then.

I guess I'll have to pull from the car again. I haven't seen the front tire on my tune. That would have saved me a bunch of headaches finding tire sizes that I wanted if that was the case, oh well next time I guess


I'd like to know what everyone else has in there stock file for revs per and final drive in the fscm

[squish72]

Originally Posted by wnta1ss

Since you are changing tires, then revs/mi is the parameter that correlates. Mine I've done in the ECM file's speedo section, never tried to do anything with the speedo in the fscm.

Noticed that with beta 4.3, now have parameters for tire circumference of the driven and undriven tires. My stock file says driven is 2150mm which correlates to the 748 revs/mile in the speedo. Wondering if setting these for new tires will make Stabilitrack happy? I've put short tires on just the rear before, set them in speedo revs/mi, and the computer was NOT happy, lights flashing and car felt like it did not want to go. Couple of years ago so these parameters were not there back then.

Downloaded the newest beta and still don't see undriven size care to show me?

[wnta1ss]

This read is from my stock 2017 SS, think using 3.3 beta at the time. Back then you could not read the TCM or FSCM on the 2017, this read was ONLY the ECM. Now opening that file with 4.3 beta, I see these extra parameters. Screenshot below

[squish72]

Thanks for that. That's crazy wonder if it's because you have a 17? Its just strange to me that when using the wizard it asks for ratio and it changes only the middle one which is the fscm. Wondering if Chevy just uses that one to adjust. Any variation in tire size from car to car?

[wnta1ss]

Originally Posted by squish72

Thanks for that. That's crazy wonder if it's because you have a 17? Its just strange to me that when using the wizard it asks for ratio and it changes only the middle one which is the fscm. Wondering if Chevy just uses that one to adjust. Any variation in tire size from car to car?

I have never used this wizard that you are talking about, my screenshot was simply opening the ECM file using 4.3 beta and going to Speedo>Calibration.

You might be onto something with the 2016 vs 2017. I checked a read of a tuned 2016 SS, and it has 745 revs/mi, and I cannot see the individual driven and undriven wheel circumferences using 4.3 beta that I do see with mine. Tires are the same for 2016 and 2017 SS models, being 275/35-20 Goodyear F1 runflats. Screenshot of the 2016 below:

[arpad_m]

FYI here is what the latest HPTuners 4.4.2 version shows for my 2018 2SS. No circumference params either, but there is an additional FSCM gear ratio(?). Same display with the previous 4.3.608 beta.

[squish72]

That's interesting that wnta1ss has a different set of parameters.

I had my new tires installed and used the wizard, at first I used Michelins listed size 26.7 and it was off but then I remembered true height is affected by the weight of the car. Dropped it to 26.4 and it's close. I'll have to drop it a little more to get it exact.

Just weird that all three final ratios listed aren't the same from the factory.

[wnta1ss]

As I'd mentioned, my 2017 read that was posted earlier was just the ECM. Here is the speedo tab from a file with ECM/TCM/FSCM read of the 2017. It now has that 3.07 ratio showing like you guys saw, but still has the tire circumferences that others are not showing. Note: the 2016 file was ECM/TCM read as I recall, so that 3.07 probably comes from the FSCM like you guys were thinking?

[squish72]

Still kinda blowing my mind with this. I'll have to try just pulling the ecm just to see if mine changes at all. If you hover the cursor over the gear ratios it shows the description in the lower corner of the box and shows what module it's from.

I got my speedo very very close with using a 26.2 tire size in the wizard, I did a quick measure with a tape and the mounted tire was damn near 26 3/4" crazy that it would be that far off.

[arpad_m]

The last two hardware parameters are also missing from my tune. It is a full file, meaning it has ECM+TCM+FSCM. If some others with a 2018 can chime in, we could try and figure out if these are missing altogether from 2018's.

[KingLT1]

You use 2.77 for A8 or 3.73 for M6 and then enter the new "actual height", meaning you measure the tire loaded on the vehicle with a straight edge and input that number. it usually ends up being around .7 shorter then the manufacturer spec.

[vtirocz]

Originally Posted by wnta1ss

As I'd mentioned, my 2017 read that was posted earlier was just the ECM. Here is the speedo tab from a file with ECM/TCM/FSCM read of the 2017. It now has that 3.07 ratio showing like you guys saw, but still has the tire circumferences that others are not showing. Note: the 2016 file was ECM/TCM read as I recall, so that 3.07 probably comes from the FSCM like you guys were thinking?


Attachment 987044

I also see the driven and non-driven tire circumference for 2017 Camaro SS tune file. I wonder if correcting that will keep stability control happy? But then there's still ABS which I believe is hard coded into that module.