Thread: Calculating Tyre sizes

If a tyre has no aspect ratio mentioned, is the default 80%?
That's the figure I got calculating out a 205R16 (original rangie tyre).

Originally Posted by Dougal

If a tyre has no aspect ratio mentioned, is the default 80%?
That's the figure I got calculating out a 205R16 (original rangie tyre).

I think so if it's a metric size tyre, (something rattling around in the old noggin says 78% ??) whereas the old imperial sizes, 6.00-16, 7.00-16, 7.50-16 were close enough to 100%

Originally Posted by rick130

I think so if it's a metric size tyre, (something rattling around in the old noggin says 78% ??) whereas the old imperial sizes, 6.00-16, 7.00-16, 7.50-16 were close enough to 100%

My memory says 85% - but I suspect it depends on the manufacturer.

The "old" imperial sizes (still manufactured so not sure "old" is the right word) were necessarily 100% for crossply tyres and remained the same for radial tyres, but again, there are almost certainly variations between manufacturers and also manufacturer's versions of the basic dimensions vary I have 7.50 tyres from different manufacturers that are different in both width and diameter by almost an inch.

John

Originally Posted by dm_td5

265mm width
75 percent of width in wall height
R implies radial
16" rim

1" = 25.4mm

So 265/75R16 is
265mm wide
803.9mm diameter = 2 x 75% of 265mm + 16" = 31.6"

See here: Tool

That's not very nice, calling him a tool !

Originally Posted by waynep

That's not very nice, calling him a tool !