Your mileage is correct......mine does this as well. It is basically giving you an estimated amount of miles you can go for the tank based upon the mileage & driving you did with the last tank of fuel. Since you were driving on the highway you car kept recalculating what it "should" get for the tank.
For my car I have had the silly thing tell me that with my drving style that I "should" get 640 miles for the tank. Tell you what.....I can only drive nice for so long before I have to let loose & kill my mileage.......so no.....I haven't gotten over 600 per tank yet.......yet. But my overall calculations have the cars computer at a pretty accurate source.
I usually take the amount of miles driven by the amount of fuel that I put in the car at the time of fill up. Usually I do this at an early morning time when it is still cool out. I top the tank off which I know is a no-no...but I do it anyway.
As far as the clicking noise....yep....mine does this as well. I don't know if it has something to do with the fuel pump or what....but it is normal.
When my tank gets low.....mine will do the same thig with the ----. Even my wifes Altima does the same thing when it gets to the point of miles until empty, reaches less than 30 miles.
As far as the aux. jack.....I assume for your MP3 player........I picked this little gem up from E-Bay a while back for my worktruck & it works very well:
http://cgi.ebay.com/Black-LCD-Wireless-FM-...id=p3286.c0.m14I paid a little more than that....but a wireless FM transmitter works well for me. I know Monster has a few of these out as well that will run you around $75+....can be bought at your local Target store....but all you do is set your FM Radio to whatever the transmitter frequency is set at & plug your MP3 into the transmitter & whola....your set. Your Mp3 player plays through your cars radio. Maybe this wil help you out?