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Tire size question

S

Straw Boss

Well-known Member
480/85R34
420/70R34
The wheel is the same size at 34 inches and the tires look to be identical height when parked side by side, so how do you account for the different numbers? Must be a difference in side wall and width but I'm not sure how to cipher the numbers. Even if I did, metric doesn't mean anything to me. One of these tires must be a 16.9?
 
With metric tire designations the first number is the width in millimeters: 480 is about 18.9" and 420 is about 16.5". The next number is the ratio of the tire's sidewall height to its width. A lower number signifies a shorter sidewall while a larger number means a taller, skinnier profile. With your examples the 480/85R34 would be a lot taller than the 420/70R34 since it is not only wider but also has a taller aspect ratio: the 480 width tire would have a sidewall height of 408 mm while the 420 sidewall would measure just under 300 mm. This would mean that the first tire should measure about eight inches shorter overall (two times the sidewall height difference).
 
I just found where 1"= 25.4 mm.
So using this conversion and doing the math, I get the same numbers as Brendon.
480/70R34 is an 18.9" tire.
420/85R34 is a 16.5" tire.
The difference in side wall height is 8/10ths of an inch (not 8 inches) which is undetectable to the naked eye so it's basically the same height tire.

So for the math I took 480 divide by 25.4=18.9 for the width.
For the sidewall, I took .70x18.9=13.23
Did the same math using the other set of numbers on the other tire to get the difference. I think this is right. Now the question is why hasn't the tire industry changed the wheel sizes to metric?
 
Looks like in the original post you had the width and aspect ratios reversed between the two tires with the taller ratio with the wider tire. With the sizes in this post you are correct - the two tires are about the same height.
 
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