according to randmcnally.com, it's 486.7 miles. 

I don't know about the geo prism exactly, but most vehicles have the fuel pumps in the gas tank now.  I know on my 98 metro, it is. 

to replace, first spray some PB Blaster on the threads of the bolts on the straps holding the tank in place, then let it sit overnight.  Next day, spray it again, then loosen almost all the way, then put something under the tank to support it as you finish removal of the nuts.  A jack on wheels works real nice for this, cause you can move it forward or back a little bit if you need.  Reach above the tank and find the wire and hose connections and disconnect them.  then lower the tank carefully making sure there is nothing else attached anywhere.  When you have the tank down, there is an access hole on the top, on the two models I've worked on, it was on the top, and the fuel pump was attached to it, the method of keeping that access cover on differs, the two I've worked on one was screws, and the other was a lock-ring.  When you put it back together, make sure the gasket is in place and stays in place while you get the cover back on! 

Good luck!