Thinking about picking up a set of 15" x 6.5 rims for 91 Westy automatic.

The idea being to lower my final drive ratio a bit (is this right
terminology?) and make the motor turn less revs on the hwy, burn less fuel,
etc.

I want the biggest wheel/tire combo in the rear that the world has ever
seen . . .
(but I will settle for something that works.)

Will the 15's with 75 series tires fit under there?  Anyone have any
experience with this experiment?  Been laying awake at night wondering
about the rev reduction.  Then it's off to dreamland.  Wake back up, it's
rev reduction hypothefication.  Twisting and turning.  (No pun intended)
You can see why I need help . . .

Thanks

HK