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Everyone is probably aware of internet resources such as BikeInsights.com that allow the user to compare two different frames/bikes' size and geometry both visually and through raw data.

I have looked but failed to find a similar tool to compare two different tyre sizes. For instance, if I wanted to compare the outer circumference of a 26 by 2.2" tyre with a 700 by 33, does anyone know of a tool that would give me perhaps the visual comparison and list the diameter and circumference for each?

Tools like this exist in the automotive world, allowing people who want to fit fancy alloy wheels to their car to choose a tyre & wheel size that will be compatible with the clocks, for istance or clear the wheelarch.

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  • Would such a tool work in the biking world? For example the circumference of your 700x33 tyre would change depending on if it was treaded or slick. It would also change depending if you mounted it on a 17mm internal vs 25mm internal rim width. And finally each manufacturer seems to have VERY different rulers with tyres supposedly the same size coming out very different sizes. Some manufacturers can't even make their own sizing consistent!
    – Andy P
    Jun 20, 2022 at 19:11
  • It would work if I was considering putting a 700c wheel into a frame designed for 650b or 26" wheels: how narrow would I need? Making a 650b conversion on a 700c frame: how much lower will the bottom bracket be? There are many similar uses. Thanks @AndyP
    – Noise
    Jun 20, 2022 at 19:13
  • I voted to close because this is effectively looking for a service recommendation which is off topic. However I think I found what you are looking for: bikecalc.com/wheel_size_math
    – Andy P
    Jun 20, 2022 at 19:24
  • @AndyP You are wrong, I am looking for a simple method and/or software, either web based/html or not, both of which Nathan has supplied in his answer. Your link was also a good tool, thanks.
    – Noise
    Jun 20, 2022 at 20:12
  • Software, web based or not, is a product or service so falls into the off topic category. Honestly, I think this excludes a great many good questions and answers (like this one), but they're not my rules. Hence why I provided a half answer via the comments (which is also not technically allowed.... )
    – Andy P
    Jun 20, 2022 at 20:32

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One of the gotchas with doing this without measuring specific tires is that the thickness of the tire at the ground contact is variable enough to screw things up, depending on the level of accuracy needed. If not for that factor, you get pretty close to being able to make accurate predictions just by doing BSD + 2(tire width) = approximate outside diameter of inflated tire.

In other words, some tires are thick enough at the contact area (puncture protection etc) that they come out more egg-shaped in cross section than the above would predict. Panaracer RiBMOs are an extreme example. But for most kinds of lighter tires like most road/gravel tires, the above works most of the time.

I've seen a few group spreadsheets and around in the framebuilding and rando worlds that database the exact measured inflated OD of certain popular tires. BikeCAD also has a database built in (since needing an exact inflated OD measurement is critical to designing frames). BikeCAD actually recommends against using the BSD+width+width method above, but it's often been good enough for my purposes when needing a fast off the cuff can-you-switch-this-to-that answer. It will at least let you know by comparison which combinations are too far off to consider.

Framebuilders when needing to answer the kinds of questions that this data is useful for will often tend to base things on firsthand measurements of the inflated ODs, because getting it wrong even by a little is problematic in that world.

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  • For clarity - how well, if at all, does this work for the likes of MTB tires?
    – mattnz
    Jun 21, 2022 at 0:41
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    It basically holds. For example, 27.5x3 and 29x2.4 reliably interchange on 27+ frames, and they math out to being like 1-2mm different in radius if calculated this way. Jun 21, 2022 at 1:55
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    It's a little fuzzy, but to avoid that completely one really needs a tire specific measurement. Jun 21, 2022 at 1:57

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