i once had 175/70 x 13's on my 1200 sedan,they were slightly taller than stock,so increased the gearing,but not by much.the drop in diameter will affect the gearing,but you could always get a different diff ratio.if this is a h165 diff in a later ute housing,series 1 bluebirds had a 3.89:1 ratio,& maybe even a 3.7:1 ratio.if it's an early 1200 ute,you could always use the 3.9:1 from a 1200 sedan.