It's time for new tires on the R100. If I remember correctly, my dad (the previous owner) put on an oversized rear tire. That's great and all, but I want 100% factory size when it comes to tires and tubes. The problem is everything I find in the manual is in inches, but shopping online, everything is metric. I can't seem to figure out the conversion, so any help would be appreciated. Any brand recommendations? My bike: 1978 R100/7T. Build date 10/77 Wheels: Stock spoked 1.85X19 fronts and 2.15X18 rear. Tubes: Unknown? Here's what the manual says, but, of course, it's in inches again! argh..