ThisLifeILead

Howdy guys and gals, i finally found a suitable replacement for my 93 yota 4x4.
Picked up a 2006 access cab taco. TRD SR5 OFFROAD with the locking diff ooo ya. 130,000 Km's. (80k miles).
The chick i bought the truck from just put new Hercules trail digger M/T tires on the truck but they are teeny tiny. P245 75R16's....they look small for the unlifted truck. The problem im experiencing is a rumble when im coasting to a stop from hwy speeds. Im assuming its the tires as they are noisey as hell compared to the duratracs or hankook dynapro's ive had. If i shift into neutral while coasting the noise doesnt change so im sure its not driveline related. Just looking from some peace of mind with these tires.
Also, why would someone want skinny offroad tires? ive never known whats the deal with that as i see guys rocking skinny offroad tires quite a bit here in the northern prairies. does it help with fuel economy on the hwy?