tire codes - can anyone explain them?

[J Charlton]

I don't think that I'm the only one (maybe I am) who does not understand the logic behind the size codes for tires.
EG - 225/75/15 well the 15 is obvious, but what do the other two numbers actually represent?
Someone gave me this link to a neat tire calculator
http://www.miata.net/garage/tirecalc.html
Its great to play around with and gives me dimensions but the more I use it, the less I understand about what the first two numbers of the tire size represent or their relationship to each other.
Thanks folks - I'm sure that I speak for many.
JC
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NAHT hightop availability May 18 2023 -
Bend Oregon - for Oregon, California- (7 tot , 3 available), Kennewick Wa (6 tot, 1 available), Small Car Performance Fife Wa. (7 tot 4 avail ), Fairbanks Alaska (1 tot 0 avail)
Future availability TBD : Springfield Mass. Staunton Va, Florida, Colorado, Grand Junction Co., SLC probably late 2024

[JunkYarDog]

225/75/15=

225=mm wide

75= height is 75% of the width

So 75% of 225 mm gives you 168.75mm sidewall height

15=as stated is rim size
_________________
I have never owned a VW I didn't like, but there have been a few Fords and Chevy's.
85' GL Sunroof
68' Beetle sedan
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(67' Volvo 122S pending)

[J Charlton]

Thank you for the succinct explanation. I guess the confusion is partly due to the mixing of measurement units for the same object.
Well, we have a size - the width in mm, the next number is a ratio, and the last is a measurement in inches. Who thought this one up?
Anyway - thanks again - I guess we can work with anything as long as we know what it is.
_________________
NAHT hightop availability May 18 2023 -
Bend Oregon - for Oregon, California- (7 tot , 3 available), Kennewick Wa (6 tot, 1 available), Small Car Performance Fife Wa. (7 tot 4 avail ), Fairbanks Alaska (1 tot 0 avail)
Future availability TBD : Springfield Mass. Staunton Va, Florida, Colorado, Grand Junction Co., SLC probably late 2024

[Amskeptic]

[buildyourown]

Yeah, it is archaic and confusing, but that's the system we got.
That's why we use a calculator.


It's not that hard. Your wheel size is fixed, assuming your not buying new wheels.
Your width is a bit flexible based on what fits. 205-225 is the norm.
Pick an aspect ratio that gives you the dia you want. Remember, if you go too big with a stock motor, your van will feel very slow.

[Wildthings]

I now run a set of 235/75R15LTs on a vehicle that I once ran H78-15s on. Over the decades it has also had 8.55-15s, 7.00-15s truck tires and originally came 7.60-15 tires when new. These are all essentially the same size tire which supersede one another. So what's so complicated about the metric designation system?