Sorry, but it's not an AWD system in the manner you thinking.
The NV247 is an on-demand 4-wheel drive transfer case with two operating ranges and a neutral position. Under normal driving conditions, the system operates conventionally, and the majority of available torque is applied to the rear wheels. However, when front-to-rear wheel speed variations exist, the progressive differential transfers torque to the axle withthe better traction, thus minimizing wheel spin and maximizing .
The 16/18 in city and 20 highway is certainly wishful thinking, probably around 16/17 overall mileage with the '01 and up with the 5 A/T. Great T/C, bur doesn't like anything other than the 247 fluid, and likes changes @ every 30,000 miles. Actualy the 242 with the selector in PT, the mileage wiill be worse as the front and rears require more power to operate continuously.