It really gets interesting when you have failed to remove the tapes over the pitot heads......which I have done not once but 3 times!!! (in a glider on aerotow). The ASI initially registers zero (20 knots, really, as the previous posters have mentioned). It then moves round the dial, completely round the dial, until the needle has gone as far as it can go! So I was now IAS 250 knots, but still following the same tug aircraft at the same old speed I took off at.
So sometimes don't trust the instrument! Actually when you get used to your aeroplane, you don't actually NEED instruments in VMC anyhow. Nice to have, though.