I ride a 48/19 with 165mm cranks, which is a bit higher geared than your 42/17 even with the shorter crank (overall gain of 5.2 compared to 4.9). This can just about cover a speed range for me between about 10kph and 42kph. I can get up 10% OK without grovelling and weaving if it's a very short section (like a few 100m). The trick is to spin up as hard as you can just before the hill kicks to charge your "kinetic energy battery" and then gradually lose cadence as you go up. I can ride up a longer 5-8% reasonably OK with this gear. If you go much lower than this getting down the same hills becomes a problem. Unless you use a freewheel but for me that's much less fun!