I take it you have changed the OE silencer? 18" wheels regardless of tyre will foul it on 3/4 compression. The actual maximum diameter depends to a degree on what aftermarket exhaust you run. All the single-sided systems will be fine, but be wary that the 'Y' pipe arrangements on some twin system will still get in the way of an 18" wheel.

On a 18 x 2.50 rim, a 120/90 is the default tyre size.

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