SCT BDX showing Tire Size in Inches

[mdmayberry09]

New to tuning for the GT and just a bit confused. When I got my tune, I mentioned that my tire size is 305/35ZR20 because my speedometer has been off since picking them up. Noticed that after the tune, it's till off, so was going to update it myself using the SCT BDX, which is my tuning device. When I went to change it after following/watching a few "how-to's" and finding my REV/MILE number, my SCT BDX is showing tire size in Inches, and saying my tire is 163", rather than something like 710 rev/mile. How do I find this random inches value, rather than rev value?

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[BigR4444]

You're 28.4 inches.

The way you would actually calculate this is; *deep breath*
305/35 means the tire is 305mm wide, and the aspect ratio, or the thickness of the tires sidewall is 35% of 305... or 106.75.
So multiply that by two, because if you're measuring the tire from one end to the other you're crossing that sidewall twice, which is 213.5mm.
213.5mm is 8.406 inches; plus the twenty inches of the rim diameter... so 28.406 inches....

...See that's easy!

 

[mdmayberry09]

You're 28.4 inches.

The way you would actually calculate this is; *deep breath*
305/35 means the tire is 305mm wide, and the aspect ratio, or the thickness of the tires sidewall is 35% of 305... or 106.75.
So multiply that by two, because if you're measuring the tire from one end to the other you're crossing that sidewall twice, which is 213.5mm.
213.5mm is 8.406 inches; plus the twenty inches of the rim diameter... so 28.406 inches....

...See that's easy!

I had actually used this yesterday, but when I saw the 28.4 value, thought either I was very wrong somehow even though I knew the calculator was showing correctly, or that the tune provided was exceptionally wrong. Mostly just confused where they're getting their 163 inch value from so didn't want to change it without checking first, because even with it having 163 inches set as the "tire size" it's only about 3mph off at 70+ mph, which added just more confusion haha. I thought at first, maybe they're doing some sort of weird measurement, but then math kicked in and I realized that 163 inches is almost 14ft and I don't think any part of the wheels are 14+ ft, even if you were to cut the tire and lay it flat.

 

[mdmayberry09]

I have also opened a ticket with SCT and AM (where I purchased), but with the holiday weekend, thought I'd check here as well in case anyone had the same issue. Search didn't have many/any results, so doesn't appear to be a common issue.

 

[mdmayberry09]

Well, they ended up confusing me even more. Told me to take the overall height (28.4 inches or 721 millimeters), not sure if they meant diameter or circumference so awaiting clarification, divide that by 3.14, then divide that into 5280, which made the numbers still completely off. So something isn't communicating through from support to me, or me to support. I tried all variations of the math that I could think of, including inches and millimeters, but didn't even get a value close to their 163 inch value they provided. I thought maybe they meant circumference when they said "height", but even that doesn't ever obtain their 163 in. value, but I did get 185.85" as the value for my current tire when doing it that way, so I just asked for further clarification on how they obtained their number.

Formula Provided:
Y = Tire Size for SCT BDX Tuner
Total Height / 3.14 = X
5280 / X = Y

All the ways I tried to make their formula return a value close to what they gave me in the tune:

Tire = 305/35R20
Diameter = 28.4 inches or 721 millimeters
Circumference = 89.2 inches or 2266 millimeters

Using "Height" (diameter):

1. 28.4in/3.14, then 5280/9.045 = 583.75
2. 28.4in/3.14, then 9.045/5280 = 0.0017
3. 721mm/3.14, then 5280/229.62 = 22.99
4. 721mm/3.14, then 229.62/5280 = 0.0434

Using Circumference:

1. 89.2/3.14, then 5280/28.41 = 185.85 (closest value yet)
2. 89.2/3.14, then 28.41/5280 = 0.0054
3. 2266/3.14, then 5280/721.66 = 7.32
4. 2266/3.14, then 721.66/5280 = 0.1367

I believe #1 circumference is correct, but their number is all that is throwing me off still, but mostly concerning to me, because even with OEM wheel size for 2021 models with the 5.0 460hp option (255/40R19, 235/50R18, 305/30R19, 265/35R20, SRC: Ford Mustang 2021 - Wheel & Tire Sizes, PCD, Offset and Rims specs - Wheel-Size.com) the values do not match theirs, which makes me worry they're just tossing values in to get tunes out as quickly as possible, without verification or review.

255/40R19 = 195.27 (84.9/3.14, then 5280/27.04)
235/50R18 = 196.70 (85.6/3.14, then 5280/27.26)
305/30R19 = 201.44 (82.3/3.14, then 5280/26.21)
265/35R20 = 193.48 (85.7/3.14, then 5280/27.29)

Will update once I have further clarification from the tuner about the value they chose to use.

 

[BigR4444]

Some of the things they're saying aren't making sense to me.

I have an SCT Rev-X; all I had to do was go
to special functions and select "tire diameter"
My stock diameter was 27" and I changed it to 28.4. (My rear tire is the exact same size as yours)
I would think the settings would be the same since they're both SCT...

As for that "3.14" they threw at you, the only thing that would make sense is that they think you have the circumference and not the diameter.
In the shot I sent to you, as you see, the tires circumference is 89.2... if you divide that by 3.14 you get... 28.4

I don't understand where that 163 came from. I think they made a mistake at some point, and now they're confusing you because they don't realize they made the mistake. They're giving you some sort of formula to calculate revs per mile or something.

If the setting says "tire diameter" then you put in the diameter of your tires in inches. We've established that they're 28.4 inches.

Don't just assume they're flawless, and you're the one making the mistake.

 

[Rick#7]

It's been awhile since my basic geometry class (like 40 years), but I don't think you divide by pi (3.14), if you multiply the diameter by pi you get the circumference, or 89.2" in your case. 5280 is 1 mile in feet, so if you divide the circumference inches by 12, then divide that into 5280 you get the revs per mile, 709.7 for your tire size.

None of that has any value close to 163, and dividing the diameter by pi has no mathematical purpose, so I have no idea what the formula they gave you is supposed to show, and there definitely is some clarification needed. The value of 163" for tire diameter in your tune makes no sense either, but maybe if the number is outside a preset range in the programming the tune uses a default value for it, i.e. the stock tire diameter , but that's just a guess.

Sorry I can't shed any more light on the confusion, but let us know what they say, I'm inclined to think someone there is confused or made an error, and is just trying to cover their butt by confusing you

 

[mdmayberry09]

Some of the things they're saying aren't making sense to me.

I have an SCT Rev-X; all I had to do was go
to special functions and select "tire diameter"
My stock diameter was 27" and I changed it to 28.4. (My rear tire is the exact same size as yours)
I would think the settings would be the same since they're both SCT...

As for that "3.14" they threw at you, the only thing that would make sense is that they think you have the circumference and not the diameter.
In the shot I sent to you, as you see, the tires circumference is 89.2... if you divide that by 3.14 you get... 28.4

I don't understand where that 163 came from. I think they made a mistake at some point, and now they're confusing you because they don't realize they made the mistake. They're giving you some sort of formula to calculate revs per mile or something.

If the setting says "tire diameter" then you put in the diameter of your tires in inches. We've established that they're 28.4 inches.

Don't just assume they're flawless, and you're the one making the mistake.

I'll try entering 28.4 value just to see if for some reason, the value there now is a place holder. I'm not entirely sure why they gave me the value/formula they did, and on top of that when I replied back saying what I found and they said "hey cool, give it a try and let us know." so probably going to just remove whatever tune they made for the time until I can get a more reputable one than the Bama tune, because who knows what else is just "we'll give it a try and see." that's hidden in the tune.

 

[mdmayberry09]

And for full transparency, to make sure I'm not misunderstanding or reading incorrectly, here is the reply/formula given to me as mentioned. Bottom message is 1st, top is the reply.

 

[BigR4444]

Well I have a 93 octane Bama tune and I love it.
When I got the new Wheels and tires, I emailed them and asked for a revised tune with the new tire size.
They sent me a new tune... and the tire size was still 27 inches

I just went back to my original tune, and changed the diameter myself.
And when I switched out my rear gear; I didn't even bother emailing them.
I just went into the special functions and changed the setting to 4.09 myself as well.

Maybe they're really good and selecting components related to intake and exhaust when dialing up a tune for customers, but other features aren't their strong point.

 

[BigR4444]

And for full transparency, to make sure I'm not misunderstanding or reading incorrectly, here is the reply/formula given to me as mentioned. Bottom message is 1st, top is the reply.

Well, looking at that, perhaps that could be taken the wrong way; he may have been confused by the way you phrased, "it's showing in inches rather then revs per mile" ...like he thought you needed to calculate revs per mile.
I think whoever wrote the tune accidentally put the 163 in there. Never know, it could have simply been a error where his mouse was hovering over it and he was looking at something else and accidentally typed that incorrect value in there

 

[mdmayberry09]

Well, looking at that, perhaps that could be taken the wrong way; he may have been confused by the way you phrased, "it's showing in inches rather then revs per mile" ...like he thought you needed to calculate revs per mile.
I think whoever wrote the tune accidentally put the 163 in there. Never know, it could have simply been a error where his mouse was hovering over it and he was looking at something else and accidentally typed that incorrect value in there

That's what I thought was a possibility, but even then revs per mile are around 710 for the size, so I'll just give 28.4 a shot and see what happens, I'll know pretty quick if it's off from the value they entered as default lol

 

[mdmayberry09]

Well, I'm about over the SCT Rev-X and to the point of returning it. It won't allow me to update the wheel size, the axle ratio, the TPMS settings, or anything. I go to update the wheel size, it shows the 160+ value, says to enter a value between 23-40, enter 28.4, goes to axle ratio, shows a 400+ value, says to enter a value between 1-5, enter 3.55, car electronics flash quickly and says it failed. Contact Support. I've contacted them, so we'll see what their answer is to this issue, but it did the same for the TPMS. TPMS on the tune is set to 0.0 by default for front/rear, go to update them and it immediately fails and says contact support. Either they've locked the option to change this, or the device is faulty, either way, waiting for support to answer for the time.

I can tune the car, so I know the device and vehicle can talk, I just apparently cant change anything important outside of the tune.

 

[mdmayberry09]

Well, they revised their tune, but no change. Parameters for wheel size on the SCT BDX are 23-40 but their value still shows 163.48in, parameters for axle ratio are 1-5, theirs still shows a 494.08:1 value, parameters for TPMS are 30+ and theirs are set at 0. I still cannot change these manually.

After the revised tune, speedometer is still off as well by about 4-5mph at highway speeds after they said they fixed it.

I sent a follow up to them to ask why these values are still showing incorrectly, why I cannot change them manually since that's one of the key features of the tuning device, and why their parameters are showing so far outside of the range the device should even be able to handle, they've gone radio silent so far.

After this, I'm almost to the point of reverting to stock and returning the device for a different brand, or picking up a tune with a different company. I don't believe the device itself is to blame, a lot of the "support" i've gotten back from bama has been "give it a try and let us know." other than the tune revision that changes absolutely no parameters.

Speedometer issues
Speed showing -> Actual Speed
65mph -> 68mph
70mph -> 74mph
75mph -> 79mph
80mph -> 85mph
85mph -> 90mph

Stock Tire size showing as 163.48in, current tire size showing as 163.48in, should be 28.4in
Stock axle ratio showing as 494.08:1, current axle ratio showing as 494.08:1, should be 3.55:1
Stock front/rear TPMS showing as 0.0 PSI, current front/rear TPMS showing as 0.0, should be 30 PSI

Again, pics included of the device showing tune name and parameters, just for transparency and showing that it's on their 93 perf 7-06 tune, which is the revision.

Errors given when I try to change myself, no error code given for troubleshooting, Bama didn't answer my question asking why I cant change it, just revised the tune, then did not answer again when I asked again after the revision.

 

[BigR4444]

Yea, that's not normal. There's something going on there...
Could be the handheld is defective or something... or maybe there's something off in the communication with your car.

I remember when I first got it, I had to connect it and give them multiple computer codes off my car. Maybe one of those codes is off or something.
But it's definitely something on their end or the actual device. You SHOULD be able to change that stuff.

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