I'd like to carry a foldable 700x37 tire as a spare, and the only one I've come across in that size is the Continental Contact Extra Light. Anyone tried that tire or have other recommendations? FYI, this is for a tandem with a heavy team, and I'm currently rolling a pair of 700x37 Continental Top Touring 2000 tires. Thanks!

I wouldn't recommend the Extra Light unless you are willing to carry multiple spare tubes. 8 or 10 per day would probably be a safe number. I had 4 blowouts - not flats but explosions - at 90 psi in the same width on a loaded touring bike and only one person. I eventually got the bike to stop having blowouts by lowering the pressure to around 70 psi.

Keep looking.

Supposedly the Conti Travel Contact is available in a folding version. If the Schwalbe Dureme has as tough a casing as the Schwalbe Extreme I think that would be a good spare for a tandem.

I toured on a single bike on 700x37c Conti TT2k for years. They work well and have a little tread that can be helpful at times. Lately I have discovered Conti Sport Contact in 700x37c. They are essentially slicks, are very fast, at least as fast as the TT2k, weigh about the same as the TT2k and have much better built in flat protection. They are also currently in production, which I don’t believe you can say about the TT2k. I have not yet tried them on one of my tandems, but they seem to work well for me. I weigh 260 lbs.

As for a folding spare, I still use an old Michelin folder that is now over 20 yrs. old. Current in-production tires might include Vittoria Randonneur Pro in 700x35c. It is a folder that has been well spoken of.

I just 'pringle' a steel bead tire, pulling the beads close together, it drapes over the rear panniers
the the stuff sacks go on top of it, and strapped down..

If you wind up not finding what you want...
It is also easy to do the three ring trick with steel bead tires. They wind up fitting nicely in even a small pannier and my cook pot nests nicely inside the loops.
https://www.rideyourbike.com/tirefold.html

Vittoria Randonneur Hypers come in 700 x 35 and 38. They are foldable, reasonably light and get great reviews for durability, flat resistance and rolling resistance.

I ride the Extra Lights, and have had good results. Granted, I'm on a single, and never run more than 70psi, so take that into consideration. Another good tire to consider is the Vittoria Randonneur Hyper.

If the bike has a load on it, 70psi is probably max. Continental is very serious about the 85psi max pressure ratings on these tires in a 37mm width. DO NOT EXCEED IT! There is no wiggle room. I won't use them again because they left me busted flat in Atlanta, TX.

Marathon Supreme is a high quality foldable tire, lightweight, rolls great.

That's what I do.

I'll go on record as saying the flat resistance of the Hyper isn't that great. It's not terrible, by any means, but doesn't seem to provide very good protection against small, sharp objects (ex: office staples). For that, you want the Randonneur Pro. It's a bit heavier that they Hyper, but seems to have better puncture resistance in my experience.

Thanks for the feedback everyone. Finding tires in stock locally has been a challenge, but I managed to track down a Schwalbe Marathon Supreme, so I'm going with that.