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  #1  
Old 09-21-2021, 08:12 AM
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Default Tire sidewall math

Hey everyone,
I am a little stumped on tire sidewall dimensions.

If a 295/40-18 has a 4.6 inch sidewall, does that mean the amount of tire showing is 4.6", or does it include the bead hidden in the wheel?

Thanks!
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Old 09-21-2021, 08:24 AM
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An 18" wheel actually measures a little over 19" in diameter. Actual visible sidewall on that tire would be approximately 3.85" depending on rim design.
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Old 09-21-2021, 08:26 AM
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Thanks Donny!
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Old 09-23-2021, 05:28 PM
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Hmmm those #s seem way too high in my opinion. I have found this:

https://tredwear.com/dealer-application/

to be very accurate.

Just plug in width mm (295) and Height % (40) and click on calculate - for any diameter (17, 18, 19 etc) a 295/40 tire your "usable" sidewall should be around 2.4"

For a sanity check since this is MUCH lower then what you guys are saying. Here's a pic of my old 305/45/18s w/1" tall glued on RWLs. If you plug in the 305 and 45 and calculate you will see it shows 3.15" Here's a close up:



If those letters are 1", I think 3.1" looks correct just eyeballing the pic and there's no way a 295/40 is going to be more than 2.5" of sidewall at the max.

-Joe
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Old 09-24-2021, 10:05 AM
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A Falken RT660 295/40-18 has an overall diameter of 27.4"

Even if the face of an 18" rim measures 20" that still leaves 7.4"/2 = 3.7"

That calculator doesn't give you visible sidewall height It gives you available sidewall height on which you can put a sticker on successfully.
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Old 09-27-2021, 08:27 PM
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You should also keep in mind that sizes aren't exact even though there's supposed to be math behind it all. For instance, not all 315's are the same width. I use Tirerack and Discount Tires website to check overall diameter of a tire I'm interested in to confirm it's what I am expecting. You'll also need to look at the tire and maybe read reviews if you're wanting a squared off look or a rounded look to the tread and sidewall intersection. It's a pain, lol.
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Old 09-28-2021, 08:27 AM
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X2 with Trey .. its quite surprising the variations in the "same size" tire from manufacturer to manufacturer
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Old 09-28-2021, 09:10 AM
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Thanks guy's I am mocking up ride height and even if it is 1" off I should be good. I haven't decided on a 295 or a 305 yet. I'm also a couple of years or more from finishing my car so I don't want to get tires yet, not sure how long they last sitting in the garage past there made date.
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Old 09-30-2021, 07:37 PM
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They'll last for quite some time really if they're stored inside and out of the sunlight. With that said, I'd hold off until you're closer to actually needing them if you can. There's no telling what new technology or designs will be released soon.
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Old 10-02-2021, 08:10 AM
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Quote:
Originally Posted by jarhead View Post
Thanks guy's I am mocking up ride height and even if it is 1" off I should be good. I haven't decided on a 295 or a 305 yet. I'm also a couple of years or more from finishing my car so I don't want to get tires yet, not sure how long they last sitting in the garage past there made date.
i see a lot of used tires on FB marketplace, you might be able to grab some mock ups/rollers on the cheap.
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