While I dont have the Montero 3.5L V6, I do have a 3.5L V6 Monty Sport and I am getting factory rated fuel consumption (18 mpg hwy, 16 mpg city) on stock wheels (255 75 r16 if i remember correctly). My FSM states I should only use regular unleaded though (good for me) - I can make 500kms with about a quarter tank to go. Since ChrisJ posted 2.6 no's, my 88 with weber can do about 500kms but close to empty on stock wheels. I was thinking of going to larger wheels but may have to reconsider...

'88 Raider 2.6L - SW Hubs, Weber 32/36, Off-road Pkg - Dead
'88 Monty 2.7L - 5" Lift, Weber 38, 33's, Dual Fans, Dual bouncy (more goodies to install) - Still Kicking.
'03 Monty Sport - XLS, BFG A/T's - Dead