Totally achieveable. '03 4.4 X5 here, at 70/75 mph I have managed almost 600 miles GPS indicated (the ODO is generous on miles traveled) on a tank. I do the 1100 mile run (GPS again) between FL and TX on 1 stop regularly with 40 miles to E showing on the OBC upon arrival. Zero oil consumption engine but I do mix 10 ozs of Lucas upper cylinder lube in each tank. I laugh when I see the commercial for the Jeep Cherokee and they BRAG how it can go 500 miles on a tank.

__________________
'03 X5 4.4 Sport, last of the M62s (8-03 build date)
I believe in deadication to craftmanship in a world of mediocrity!