my girlfriend runs 42 x 18 = 63 and is able to make it up all the hills and bridges easily. keep in mind you'll spin out faster going down hills the lower the ratio. i run around a ratio in the low 70s. also remember good technique makes a big difference: building up good momentum and really attacking the first half of the hill, pulling well with your arms into the stroke and finally standing and mashing if necessary. there's good info out there on proper riding technique that makes your ride more efficient...