Join Date: Oct 2007
Location: Québec, Québec, Canada
Bikes: Fuji Touring 64cm, Peugeot Canada PB-18 61cm and winter beaters
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Bike tire sizing is an imprecise thing. For one, the actual width of your tire will vary according to rim width. More importantly, manufacturers do not always provide the real width, but rather some approximate value. Two 700x25c tires may not have the same width.
But yes, 1-1/8" should be around 28mm. Could be a bit narrower, could be a bit wider. The ETRTO size for those tires is 28-630, that is, 28mm wide and 630mm Bead Seat diameter. It is 25-622 for 700x25c tires, and 28-622 for 700x28c tires.