A little over a month ago I bought a non-folding GP4k Black Chili 24x650C tire. It's new from Conti this year and I thought maybe it would be worth trying out on TxDOT's boulder-seal. I finally got around to mounting it last night and went for a short fast group ride this morning. This tire is a little cheaper than the folding 23x650C triathlons I've been running most of the year and I was a little afraid that the slightly cushier tire might cut my speed.  It's a lot lower tpi so I expect it will be a little more flat-prone but th pic seems to indicate another layer that is maybe for flat-protection that the triathlon doesn't have. Results so far: it's a slightly cushier tire, no flats, and I can't tell any decrease in speed whatsoever. I think I'll be moving to this tire for a training/travelling tire and reserving the triathlons for racing and a spare foldable travelling tire.
 
24-650c:
http://www.conti-online.com/generator/www/de/en/continental/bicycle/themes/race/ recetyres/grand_prix/grand_prix_en.html
23-650C triathlon:
http://www.conti-online.com/generator/www/de/en/continental/bicycle/themes/race/ recetyres/GP%20Triathlon/gptriathlon_en.html