I’m to receive my new disc wheel today and was wondering if I should go with the 21 or the 19 tire. I was thinking that the extra width of the 21 could cause additional drag and/or turbulance at the contact area with the wheel. Any comments?
go for the 21
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Agree with Gary. Unless you have tire clearance issues (you shouldn’t with a 21 mm tire) go with the wider tire on the back.
Ok, thanks.
Tire you said? Is that a training disc? 
Go with the 21 
if you want to ride like bjorn, of course, you’ll go with 19s front and back inflated to a ridiculous psi. also drop your frontend so your chin lightly grazes the front tire.
Go with the tire that most closely matches the rim width of your disk.