I have a 7.3L 6 speed manual 2001 F350 4X4 DUALLY WITH A BIGFOOT Truck camper. I have 410 gears. The best fuel mileage is at 2000RPM and lower. In overdrive (6th gear) reving at 2000RPM, my speed is 62 miles per hour. My tire size is 235/65. If I increase my tire size would it be the same as having 373 gears (70 miles per hour at 2000RPM)? Would the next size tire fit on the stock rims?

 

tires would help a little with RPMs, next size up is a 265/75R16(31x10.50r16) next size after that is a 255/85r16 (33x10.50r16) or a 285/75r16(33x11.50r16)

 

Keep in mind bigger tires weigh more, so you will loose HP due to the added weight.