Let it snow, let it snow, let it snow...

[GuzziMoto]

20 hours ago, Scud said:

Being the curious sort, I went to measure. If I measure from the outer tread blocks horizontally, the diameter is only about 32". However, the tread is curved and the center of the tread bulges almost 1/2" per side. I don't have a proper caliper to measure the maximum diameter, but If you were to measure the diameter at the center of the tread, I think the expected 32.8" measurement is realistic.

Generally the measurement is taken from the edge of the tread. Even still, you will get a different measurement on a mounted and installed tire if you measure side to side edge of the tread vs vertically from the ground to the top edge of the tread. That would be because the tire deforms at the contact patch, how much depends on air pressure in the tire. For general tire size measurements people tend to either measure a tire off the vehicle or measure an installed tire but measure side to side so the contact patch deformation doesn't enter into it. But when you are measuring for speedometer calibration you actually want to include that contact patch deformation, typically. So, there are times when measuring vertically from the ground to the top edge of the tire is desired.

Being about 0.8" smaller then the published size is not bad. That is probably closer then the KO2 would have been. The KO2 tended to be around an inch or so smaller than the published size.

And thanks for measuring.

Edited January 28 by GuzziMoto

[audiomick]

10 hours ago, GuzziMoto said:

... But when you are measuring for speedometer calibration you actually want to include that contact patch ...

Without wanting to completely de-rail the thread, measuring the tyre is not the right way to go at that. One should mark the side-wall of the tyre where it touches the ground, and make a corresponding mark on the ground. Roll the vehicle forwards until the mark on the side-wall is back down to the ground, and mark the ground again. The distance between the two marks on the ground is the real rolling diameter of the tyre.

[GuzziMoto]

15 hours ago, audiomick said:

Without wanting to completely de-rail the thread, measuring the tyre is not the right way to go at that. One should mark the side-wall of the tyre where it touches the ground, and make a corresponding mark on the ground. Roll the vehicle forwards until the mark on the side-wall is back down to the ground, and mark the ground again. The distance between the two marks on the ground is the real rolling diameter of the tyre.

Perhaps, but most applications I work with that allow you to calibrate a speedo for different tire sizes use diameter or radius of the tire and not circumference. Perhaps others want the rolling circumference, but not ones I work with. My Jeep, for example, uses diameter. To get my 37" BFG KO2s to read the right speed and miles I had to enter a diameter of 35.25 as I recall. I have worked with bicycle speedo's that used a rolling diameter based input but I have not used rolling diameter for any automobile application I have worked with.

[p6x]

A video that seems to be dealing with facts rather than illusions;

I have my Ford Transit Connect equipped with all season tires, because in Texas, you need to be prepared for anything nowadays....

This review is excellent, even if missing the autonomy/durability concept.

Edit: Found a review that states the wear out distance;

 

Edited Thursday at 03:01 PM by p6x

[Lucky Phil]

On 1/29/2025 at 10:37 AM, audiomick said:

Without wanting to completely de-rail the thread, measuring the tyre is not the right way to go at that. One should mark the side-wall of the tyre where it touches the ground, and make a corresponding mark on the ground. Roll the vehicle forwards until the mark on the side-wall is back down to the ground, and mark the ground again. The distance between the two marks on the ground is the real rolling diameter of the tyre.

Why would you want to go through all that? Why calibrate a speedo? All three of my cars the speedos over read different amounts. The Kia 5kph@100, the Mazda 2kph@100 and the Supra 2kph@100. All calibrated off the GPS. I adjust the cruise control speed accordingly and I know at lesser speeds the difference is even smaller. Seems to me when you fit significantly different wheel sizes you just do a 60 mph drive and check it against the GPS and remember the offset and you're good. Even with my addled brain I can remember the speedo inaccuracies in three different cars. Add to that a motorcycle with a Veglia speedo!

Phil 

[Tomchri]

Happy I grew up with with not much concern about speed limits, FOR sure.

Ah well, Good Friday guys.

Cheers Tom.

 

[audiomick]

4 hours ago, Lucky Phil said:

... All three of my cars the speedos over read different amounts....

Of course. The law in Australia and in Germany, and probably most other countries, stipulates that the speedo must not show less than the actual speed. Over-estimating is fully legal, and the norm. One can assume that up to 10% over-estimation is normal. The only speedo that I have experienced that is almost accurate, i.e. almost no over-estimation, is the one on my Breva 750. It is only a km or so over at 60 km/h, and maybe 3 at 100 km/h.

Why go to the trouble? Calibrating against the GPS is the logical solution. Easy, and easy to remember for multiple vehicles. But if one is installing a new speedo and wants to calibrate it, one must measure something to get a "base value". The method described is the best way to establish the real rolling diameter of the tyre, and therefore the base-value to calibrate the speedo. I've done it a couple of times with push-bike speedos. Takes all of 3 minutes.

[docc]

And changes as the tires wears?

[audiomick]

15 minutes ago, docc said:

And changes as the tires wears?

Will mean that the speedo over-estimates, so legal.

But a meticulous owner would of course regularly check the wear on the tyres, and correct the calibration of the speedo as needed.

Or maybe just consider the idea, and have a beer instead.   

Edited Saturday at 02:45 AM by audiomick

[docc]

Pop quiz!

A) a meticulous would of course regularly check the wear on the tyres

B )  correct the calibration of the speedo as needed.

C) both A and B

D) Or maybe just consider the idea

E) and have a beer instead.

F) both D and E

[audiomick]

I'm for "F".

[GuzziMoto]

As mentioned, the typical speedo calibration I have used is based on tire diameter, not tire circumference. I would not be surprised if some tire diameter calibration software requires you to know the rolling diameter / circumference of the tire. But for our Jeep, for example, it wants to know the diameter of the tires. I have also seen some that wants to know the radius of the tire, which is generally half the diameter. I have used non-automotive speedo calibration software that used tire circumference or rolling diameter. But not for automotive applications. 

I would never bother re-calibrating it to account for tire wear. I am just not that neurotic. And the software I have used is not that precise. It generally only goes down to a quarter of an inch in diameter, or an eight of an inch in radius. But the reason I have used it is when replacing tires with tires of a different size. For example, my Jeep is currently running tires that are approx 4 - 5 inches larger in diameter then the original tires (from 32" tires to 37" tires). That would be, without adjusting the speedo calibration, a massive difference. In fact, it would be a large enough difference to upset the Jeeps computer. So, I re-calibrate so the Jeeps speedo reads about right and the Jeeps traction control and cruise control systems don't freak out.

There are times when I have sort of measured the rolling diameter of the tires, for example to confirm the axle gear ratio. Rolling the tire exactly one rotation and measuring how many rotations of the drive shaft in the one rotation of the tire will tell you what the gear ratio is in the axle. That is probably not of use to most people, but if you have a Jeep and you want to know or confirm what gear ratio you have in the axles (or even that the front and rear axles have the same ratio) there is not an easier but more accurate way.

But to how measuring tire size is relevent to this thread, the question originally came up not for calibrating speedo's or anything like that. It was about how most tires are not exactly the size listed on the tires sidewall but rather most tires are to some degree or another smaller then the listed size. And some tires / brands tend to run more smaller then others. Like BFG KO2's, they tend to run upwards of an inch and a half smaller then the listed size (a 35" KO2 tends to actually be around 33.5" in diameter). And since Scud had measured his KO3's I was comparing their actual size to the listed size to see if they also run as small. From what Scud measured, the new KO3 does not seem to run as small as the KO2 it replaces.

[audiomick]

3 hours ago, GuzziMoto said:

... the radius of the tire, which is generally half the diameter.

It would indeed be an interesting tyre if the radius wasn't exactly half the diameter.

 

Despite that, thanks for the explanation.

[gstallons]

now that was funny