Why are tire sizes stupid?

"Rainbow" (rainbeaux)
03/25/2015 at 16:06 • Filed to: None

Seriously, it doesn't do me any good to know that the sidewall height is 65% of the width of the tire. Why can't it just tell me that the tire's outside diameter is 20" or whatever the hell it is?

DISCUSSION (17)

You can tell a Finn but you can't tell him much > Rainbow
03/25/2015 at 16:07

If things were logical engineers couldn't lord their superior knowledge over you.

Smallbear wants a modern Syclone, local Maple Leafs spammer > Rainbow
03/25/2015 at 16:08

So people who know how to decode it can laugh when things go wrong for those who don't.

CKeffer > Smallbear wants a modern Syclone, local Maple Leafs spammer
03/25/2015 at 16:11

^^^^Pretty much this

JawzX2, Boost Addict. 1.6t, 2.7tt, 4.2t > Rainbow
03/25/2015 at 16:12

Because a tire is an elastic object. The total diameter/circumference of the tire is actually at least partially dependent on the width of the rim and the consequent angle of the sidewall, as well as inflation pressure and even rotational velocity! Therefor the diameter/circumference can only be described as a "nominal value under most conditions" where as the other dimensions can be fixed based on the dimensions of the mold from which the tire was manufactured. Engineers get all persnickety about imprecise specifications.

I agree though, a "nominal mounted diameter" measurement would be nice.

RamblinRover Luxury-Yacht > Rainbow
03/25/2015 at 16:12

There's probably a reason *other than* the fact it makes it easier for the tire maker to... estimate the amount of rubber used? Air? I'm somewhat stumped. Drag tires, I think, are sold in sizing as you describe. Small trailer tires are sold that way too, somewhat. Old-fashioned bias-ply sizing worked kind of like modern metric sizing, but in some respects sillier. However, the fact that it was based on aspect ratio vs. something at least related to internal air volume makes me suspect that there's some arcane reason like expected pressure change at temps for doing it that way.

http://www.treaddepot.com/content/conten…

Tohru > Rainbow
03/25/2015 at 16:12

that's basically what metric tires do... You commie

crowmolly > Rainbow
03/25/2015 at 16:14

Well, a lot of race/offroad tires come "the good way".

28x10.5x15 works for me

Rainbow > RamblinRover Luxury-Yacht
03/25/2015 at 16:15

Well sure, but for the consumer's sake they could at least add one more number to the sidewall. If I end up with different rims, I'd like to know that the whole package won't be throwing off my speedometer and odometer.

RamblinRover Luxury-Yacht > Tohru
03/25/2015 at 16:17

Bias ply sizing or GTFAC. Tire section grades A-F FTW

Snuze: Needs another Swede > JawzX2, Boost Addict. 1.6t, 2.7tt, 4.2t
03/25/2015 at 16:18

Thanks, I learned something today!

Lumpy44, Proprietor Of Fine Gif > Rainbow
03/25/2015 at 16:19

This could help.

traderQAMobileTestAutomationMobileBoostOn > Rainbow
03/25/2015 at 16:19

Luckily, truck tires are like that.

uofime > Rainbow
03/25/2015 at 16:19

it's not very complicated to calculate, the unit mismatch between the rim size [in] and the width [mm] is slightly obnoxious though

2[(w/25.4)*(Pr/100)] +R= overall diameter [in]

angrytetsuo > Rainbow
03/25/2015 at 16:22

also, you can buy 10 different brand tires of the same size and measure all of them and I bet you nearly all of them will be different "actual" sizes... so yeah very wacky stuff

later I'll tell you how the tread wear rating is garbage as well

CAR_IS_MI > Rainbow
03/25/2015 at 16:34

its so simple. a 275/45/17 has a diameter of 26.74 inches or 679.30 mm.

see

easy.

275 * .45 = 123.75 mm

you have 2 side walls (well one, but two if you're looking for diameter)

17 inches = 431.8 mm

431.8 + 123.75 + 123.75 = 679.30 mm

679.30 mm = 26.74 inches.

SIMPLE

Dingers Ghost, Champion Jockey > Rainbow
03/25/2015 at 16:40

I know that truck tires can be sold in inch? notation. Like 35x12.5x16.

I also know they seem to get more expensive when they're sized like that

akz > Rainbow
12/21/2016 at 19:17

Hello from 2016! (it’s awful, stay where you are) Anyway, I wondered the same thing, and I think I found a real answer. See below: https://en.reddit.com/r/cars/comments/5jnc9g/why_exactly_are_tires_measured_marketed_using/