I agree with the formula, as I've experienced it, and I've even seen the results of reduced inertial weight (flywheel and lightweight wheels) on a dyno.

The difference between 14 and 15" wheels that have the same weight, the same width tire, and the same overall diameter isn't going to be enough to worry about, though. The 15" tire may be a touch heavier, it all depends on what size and what model of tire you choose. The difference can be a pound... or it can be 3 pounds, just depends on the tire.

Odds are that you can ultimately find a narrower and lighter 14" tire, however. But, from what I've read, going with a low rolling resistance tire will pay off more than downsizing the tire.

Tires : Test Results : The Tire Rack

Even then, you're only looking at maybe 1-2 mpg difference for a typical car.