I recently converted my Brompton to a single speed, but the new wheel would only accept presta valve tubes. Pissed at myself for not thinking of this potential problem earlier, I started surfing the web in my disgust and came upon a quip which mentioned that some people converted their Birdies from 18" (ETRTO 355) to 16" (ETRTO 349) wheelsets because of the availabilty of tires in the 349 size. The poster also commented that he/she didn't have to change inner tubes as 6mm of difference was negligible.
Well I happened to have spare 18" Schwalbe presta valve inner tubes, not having touched my Birdy since 1999, so I found the box and started reading it over again. To my delight, I found they were rated all the way up to 20x1/8 - perfect since I use Continental Grand Prix tires on my Bike Friday! I then put one on my 16" rim, pumped it up, and it has held stout for the past couple of days' riding with no noticeable difference or decrease in ride quality. To say I am pleased is an understatement.
Oh the point of this thread? Well, I know many on this forum own multiple folding bikes with every range of tire size. For those who own 16" - 20" folders, it may be more economical just to buy 18" tubes instead of each individual size. Who would have thought Birdy's unusual tire size could ever be a benefit?
Sorry to those who already knew this. I thought I'd (re-) post this info just in case.