3RZ will bring mid to high 20's for mileage. Buddy with an 00 Single cab taco on 33's was getting 27-28 hwy before he swapped to a 5VZ.
That said, I'm with the other guys. Put in a new V8. Something with some power. Toy motors are great, but for the hassle of a swap, might as well get all you can.
Baby Beast 2- 1999 4Runner SR5
Baby Beast -1987 4Runner SR5-Gone, but not forgotten