Cycleway town
Join Date: Jul 2014
Location: Milton Keynes, England
Posts: 1,402
Bikes: 2.6kw GT LTS e-tandem, 250w Voodoo, 250w solar recumbent trike, 3-speed shopper, Merlin ol/skl mtb, 80cc Ellswick
Mentioned: 7 Post(s)
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Quoted: 569 Post(s)
They're almost certainly 17mm, and even if 19mm they'll be fine with a 1.5in tyre.
I wouldn't go down that route though. I first used 2.35in slicks on a basic MTB and love them, so there's no need to go crazy skinny when you're not trying to hit high speeds.