Seems to be about 3 miles from most of the launch pads to the VAB, so about 377 gallons for a one-way trip (http://www-pao.ksc.nasa.gov/nasafact/count3teaf.htm)
The crawler can travel about 1 MPH loaded; the tour guide at KSC (at least when I was there several years ago) made a big deal about the blazing 2 MPH it can do when unloaded. :)