tire sizes
Le Mans Master
Joined: Sep 2014
Posts: 5,278
Likes: 539
From: Kelowna BC
Never seen this before, but it looks like the 680 is the diameter. Metric to imperial is 25.4, so a 680mm tire would have an OD of 26.77"
So you'd have to do some math. The second number for the standard sizing is the aspect ratio; a percentage of the sidewall height in relation to the width, so a 315/30/XX (any rim size) would have a sidewall height of 3.72". 315/25.4 = 12.40" wide. Multiply that by 0.30 = 3.72" sidewall.
26.77OD - 18 rim dia. = 8.77 sidewall height for both
8.77/2 = 4.39" sidewall height
So if you have a 305/680/18...
4.39 x 25.4 = 111.4/305 = 0.365. or 37%, closest match would be between a 305/35/18 and 305/40/18
So you'd have to do some math. The second number for the standard sizing is the aspect ratio; a percentage of the sidewall height in relation to the width, so a 315/30/XX (any rim size) would have a sidewall height of 3.72". 315/25.4 = 12.40" wide. Multiply that by 0.30 = 3.72" sidewall.
26.77OD - 18 rim dia. = 8.77 sidewall height for both
8.77/2 = 4.39" sidewall height
So if you have a 305/680/18...
4.39 x 25.4 = 111.4/305 = 0.365. or 37%, closest match would be between a 305/35/18 and 305/40/18
Team Owner
Joined: May 2001
Posts: 87,367
Likes: 1,593
From: Western NY
Personally, I think you're looking at a "mis-print". By convention, tire sizes are shown as "section width"/"section height"/"rim diameter", and since it's posted in MM, it's already a metric measurement.
Le Mans Master
Joined: Sep 2014
Posts: 5,278
Likes: 539
From: Kelowna BC
I thought the same, but Google it. Seems the 680 number is commonly used with slicks.
