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Imperial vs Metric tyre sizing

Richard Pitman

Veteran Member
Joined
Jun 1, 2020
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Location
Worcester, England
I've looked at various conversion charts but am still a bit puzzled, wondering about fitting more modern tyres.

What is a 90/90 18 tyre equivalent to in old sizing? 3.25 18 ? 3.50 18 ?

Does it need a 1.6, 1.85 or wider rim ?

Apart from the Dunlop K82, I can't find a decent modern road tyre in those sizes.
 
I've looked at various conversion charts but am still a bit puzzled, wondering about fitting more modern tyres.

What is a 90/90 18 tyre equivalent to in old sizing? 3.25 18 ? 3.50 18 ? Simply convert mm to inches, 90 mm = 3.543" (width) x 90% of that/ 3.189" height/aspect

Does it need a 1.6, 1.85 or wider rim ? Follow tyre manufacturers guidelines....

Apart from the Dunlop K82, I can't find a decent modern road tyre in those sizes.

IN BLUE ABOVE.....
 
Thanks. Just googling this, it seems to be a FAQ going back several years, with no really definitive answer to it.

I get the tyre width part OK now, it's the rim width that seems to be a grey area. Looks like 90/90 MIGHT be OK on a 1.85 rim, the Michelins I was looking at don't have a rim width guideline.

One way to find out, I suppose ….
 
To partially answer my own question, just on visual appearance alone, the 90/90 18 Michelin Pilot Street rear tyre, mounted on a 1.85 rim, looks similar to 3.00 18 CST K70 pattern tyre mounted on a 1.60 rim, both in tread width and crown height. A 3.25 K82 on a 1.85 rim actually appears slightly wider, same height.

I'm tempted to pop these wheels into my CB175, see what they ride like, but that's an experiment for a week or so in the future.

EDIT Except nothing is that simple. I've fitted a CB200 chainset to these wheels, so I'll have to swap chain and front sprocket as well, if I try the above idea …
 
Whilst further researching tyres, I found this handy table on the Avon website, relates to their Street Runner tyres. Interesting to compare the 90/90 tyre with the 3.00 18 tyre, also to read their suggested rim widths.

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