FORScan and Tire Size Changes

[MidnightStars]

Followed this thread (several times over) to update my speedometer: How to use FORScan to adjust tire size - FORScan forum

My Mustang accepts the values without throwing codes, did the PCM relearning process (TCM not present), and my speedometer still reads 3 to 4 mph too fast...

Went from the stock Pirelli P Zero's @ 265/35/R20 -> Petlas Velox Sport PT741's @ 305/30/R19. OD of the tires are 2179.3 mm and 2079.7 mm respectively.
*Values derived from: Pirelli + Petlas.

I programmed in 2080 mm as the tire circumference in the BodyCM, followed its instructions, performed the PCM relearn (TCM relearn not done, as it was not present), and my car still reads as if it's going faster than actual. Does anyone have any support?

Can't ask on the FORScan forums, because I don't have an invite...

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[MiamiGT350]

Not my area of expertise, but maybe I can help a little.

I input the tire sizes into the Tire Size Calculator and I get 2177 mm before and 2090 mm after... not the 2080 mm you mentioned.

Doing the math by hand, I see a diameter of 662mm on the Petlas site, and I get the same total as you.

C = 2πr = 2 · π · 331 ≈ 2079.73434

If you click on the tire calculator's "About Results", they cite a 3% variance because the manufacturer numbers are not with the tire being compressed by the vehicle weight.

They also cite 82.3 in on this page, which is 2090.42mm. This is only a 0.5% difference from your number though. Inputting a higher circumference will make up for some of the discrepency, but I'm not sure it will account for 3-4mph.

Worth asking... tires are all properly inflated? Also, at what speeds are you going where it's off by 3-4 mph, and how are you measuring?

 

[MiamiGT350]

Using their Speedometer Calibration Calculator, they calculate that you're only off by 0.16mph at 75mph if you didn't program anything.

So I'm not sure what the cause of the 3-4mph you're seeing. I see you're pulling the Pirelli numbers of the same site as the calculator, so maybe they are baking in the weight adjustment for you?

Sorry if I'm not a lot of help...

 

[MAGS1]

This is what I show when plugging in the 2 tire sizes. The 4% variance is enough to throw things off internally. I’m not savvy with Forscan so I’m not sure why it’s still off after you recalibrated using Forscan.

Paging @Cobra Jet , he’s good at finding all the various issues and fixes for said issues.

 

[MidnightStars]

Not my area of expertise, but maybe I can help a little.

I input the tire sizes into the Tire Size Calculator and I get 2177 mm before and 2090 mm after... not the 2080 mm you mentioned.

Doing the math by hand, I see a diameter of 662mm on the Petlas site, and I get the same total as you.

C = 2πr = 2 · π · 331 ≈ 2079.73434

If you click on the tire calculator's "About Results", they cite a 3% variance because the manufacturer numbers are not with the tire being compressed by the vehicle weight.

They also cite 82.3 in on this page, which is 2090.42mm. This is only a 0.5% difference from your number though. Inputting a higher circumference will make up for some of the discrepency, but I'm not sure it will account for 3-4mph.

Worth asking... tires are all properly inflated? Also, at what speeds are you going where it's off by 3-4 mph, and how are you measuring?

Thank you for the notice about tires being compressed under load, I've completely neglected that. Looks like I'll either manually measure, or input slightly lower values (I recall the as-built value being lower than the unloaded tire) until it sits right.

Properly inflated @ 42 psi (Petlas spec). Speed read by GPS speed tester + Waze apps, along with built-in GPS module on BlackVue's 750S dash camera. Speed set to 69 mph (nice) via cruise control--all apps + dash cam read out a steady 66 mph on flat roads.

Using their Speedometer Calibration Calculator, they calculate that you're only off by 0.16mph at 75mph if you didn't program anything.

So I'm not sure what the cause of the 3-4mph you're seeing. I see you're pulling the Pirelli numbers of the same site as the calculator, so maybe they are baking in the weight adjustment for you?

Sorry if I'm not a lot of help...

Not sure why that's calculated differently, I used the page @MAGS1 posted in the following comment.

Thank you both for the comments, y'all are absolute legends.

I'll need to wait until tomorrow to update it and see what turns up.

 

[MidnightStars]

Just an update, followed the steps for the GPS based adjustment here: https://forscan.org/forum/viewtopic.php?t=6174

The PCM still throws a code for the BCM having a value out of its bounds, BUT the speedometer is spot on accurate (well, more like 119.9 w/ cruise control at 120) and I'm happy with it. Hopefully that code being up there for a long time doesn't affect anything else.

 

[GR1MxREAPER]

Has anyone figured out how to do this without a code coming up etc?? I see 50% of people get this to work perfect and then the other 50% either can’t get it adjusted properly or have to live with a check engine code. I want to change mine but I don’t want a code on my car and I don’t trust the dealer as my car is tuned and don’t want them fiddling around lol..
CAR- 2019 Mustang GT PP1 Car

Stock- 275/4019
NEW- 315/30/19

New tire is quite a bit smaller so definitely need to figure out something. Car is put away for winter in MN but hopefully can figure something out. Send me a PM if you are good at this stuff or if you have a shop in MN let me know and I’ll drive the car to the shop and do it there lol..

 

[iceman145]

Has anyone figured out how to do this without a code coming up etc?? I see 50% of people get this to work perfect and then the other 50% either can’t get it adjusted properly or have to live with a check engine code. I want to change mine but I don’t want a code on my car and I don’t trust the dealer as my car is tuned and don’t want them fiddling around lol..
CAR- 2019 Mustang GT PP1 Car

Stock- 275/4019
NEW- 315/30/19

New tire is quite a bit smaller so definitely need to figure out something. Car is put away for winter in MN but hopefully can figure something out. Send me a PM if you are good at this stuff or if you have a shop in MN let me know and I’ll drive the car to the shop and do it there lol..

I'm interested too. Put HP wheels and tires on my Mach 1 and the speedo is off (expected). Curious if anyone has the values from an original HP car...

 

[MidnightStars]

Has anyone figured out how to do this without a code coming up etc?? I see 50% of people get this to work perfect and then the other 50% either can’t get it adjusted properly or have to live with a check engine code. I want to change mine but I don’t want a code on my car and I don’t trust the dealer as my car is tuned and don’t want them fiddling around lol..
CAR- 2019 Mustang GT PP1 Car

Stock- 275/4019
NEW- 315/30/19

New tire is quite a bit smaller so definitely need to figure out something. Car is put away for winter in MN but hopefully can figure something out. Send me a PM if you are good at this stuff or if you have a shop in MN let me know and I’ll drive the car to the shop and do it there lol..

I'm interested too. Put HP wheels and tires on my Mach 1 and the speedo is off (expected). Curious if anyone has the values from an original HP car...

Entirely depends on your tire specs. The PCM has a hardcoded range for it to not throw a code about the BCM's tire value being invalid. If your tire falls out of this range (such as mine), that code will always be present, but hey, at least your speedometer is accurate now.

 

[GR1MxREAPER]

Entirely depends on your tire specs. The PCM has a hardcoded range for it to not throw a code about the BCM's tire value being invalid. If your tire falls out of this range (such as mine), that code will always be present, but hey, at least your speedometer is accurate now.

Ya I would just put it closest to my tire size and not have a light. I hate check engine lights.. the side should be in the system since the gt350R uses a 315/30/19 I believe.

 

[Angrey]

This thread is resurrected. I'll only add that I know that you can adjust in two locations, either through the tune/PCM or through FORSCAN but from what I think I remember, if you use one (like the tune) then the other is totally null and void. Something about all or nothing when FORSCAN vs tune updates. Not sure if that's complicating or contributing to some of the problems others are seeing.

 

[ihasnostang]

Ya I would just put it closest to my tire size and not have a light. I hate check engine lights.. the side should be in the system since the gt350R uses a 315/30/19 I believe.

i downloaded a gt350 tune from hptuners and opened it. Looks like their system uses a max/min and a nominal size.

I'm on different tire sizes than you all but for reference my 265/40/19 has a circumference
from tire website: 2182
from forscan: 2087

GR1MxREAPER

I'm in the Minneapolis area and have forscan on my laptop

 

[MidnightStars]

Ya I would just put it closest to my tire size and not have a light. I hate check engine lights.. the side should be in the system since the gt350R uses a 315/30/19 I believe.

This thread is resurrected. I'll only add that I know that you can adjust in two locations, either through the tune/PCM or through FORSCAN but from what I think I remember, if you use one (like the tune) then the other is totally null and void. Something about all or nothing when FORSCAN vs tune updates. Not sure if that's complicating or contributing to some of the problems others are seeing.

Use this:

I still have a PCM code (since BCM value is out of range), but no check engine light

 

[iceman145]

I did mine today with Forscan. Went into BdyCM and changed value from 2178 to 2080. I did not get a check engine light and the speedometer appears right via GPS speed.

 

[Angrey]

I did mine today with Forscan. Went into BdyCM and changed value from 2178 to 2080. I did not get a check engine light and the speedometer appears right via GPS speed.

That's what I was commenting. If there's an aftermarket tune, they can make adjustments in the ratios there, and then I think it overrides FORSCAN. If however there's no changes in the tune, FORSCAN will/should work.

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