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Thanks @mdcc_tester, that does result in 18mm drop
700x25 <=> 622+2*25=672mm
650Bx35 <=> 584+35*2=654mm
Considering that I currently run 170mm crank arms and I never had any pedal strike (currently running fixed), switching to 650B and 165mm crank arms woud result in 13mm pedal axis drop which does appear to be quite a bit even if it wouldn't be fixed anymore but with gears, would it be a problem? I assume that this discussion would be much easier with the BB heigh measurement but I do not have it yet. I reckon it would be quite better to run 38mm wide tire to reduce the drop
paulbiche-
only 9mm drop (half of wheel diameter difference)
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Is the bike recent enough that you can find a geometry chart with BB drop?
If it is in around the 70mm mark, and it's likely to be, then you'll be fine with 650x38B (there aren't any good tyres in 35).
I rode that size with a 72mm drop and while it noticed the lower centre of gravity it was more of a positive than anything else, as It really helps with confidence in cornering.
svenhöek
Lolo
Roll your own, just add twice the tyre section to the rim diameter (622mm for 700C, 584mm for 650B) and you'll be close enough. The actual inflated diameter is a bit more than that and affected by rim width and bead hook height among other factors, but after allowing for squish you end up more that accurate enough for judging the effect of tyre changes on BB drop.