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If a rim has 27.5mm outside width, should I be using 25c or 28c?

My initial thought was 28c, and indeed I ordered those when I ordered the wheels, but I now read that 28c tyres tend to measure 29/30mm (these are GP 5000 TL), and that from an aerodynamics point of view my 50mm deep rims might be rendered a bit moot if the tyre is wider than the rim.

So is 25c the right choice?  It's what I've been running for a few years, and I have no problems with comfort or pinchflats.  I tend to run 80ish in the back and 65ish in the front.

I'm a loser who's more interested in going fast than being comfortable, what size should I have?

I know the theory behind wider tyres rolling faster on anything other than glass-smooth surfaces, but then there's the aero side to consider.

What wins, on 50mm deep 27.5mm wide rims - 28c, or 25c?


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