My gas gauge behaves the same, always has. It takes awhile to register full then stays at full for too long and then becomes accurate at about a half tank. It then is very accurate down to zero. As far as tires go - you can get tkc80s in 17 inch sizes. I never tried them, but they probably work pretty well. I did put many dirt miles on Dunlop 606s and they do well on hard pack dirt but not soft stuff.