What if you can't spin?

dogsballs, at the moment this 48x17 is really good, and I can go up gentle inclines without standing, but I think that if I have to head up Pentonville Road, I'd probably walk up.......til I've lost some weight, and increased my leg strength. But its flat speed I'm after. Anyway, hills are for mountain goats http://photography.nationalgeographic.com/cgi-bin/pod/PhotoOfTheDay.cgi?day=21&month=4&year=05

I think 48x17 sounds pretty healthy really, for a masher (no judgement in that label).

I'd stick with it rather than going higher, and focus on good pedalling form - make sure you're lifting/unweighting on the upstroke, and try to get a bit of power through the stroke early (from about 11 o' clock and then over the top of the deadspot).

You can get an extra mph or two, without any perceived increase in effort, or physiological distress.