Best tire size for high mileage?

I have the original ConitouringContact 195 65 R15 tires on the original steel wheels. So, let's say that I want to increase my final gearing in order to get better mileage. What is the best and most cost effective way to increase the overall tire diameter(preferably without requiring wheel well modification). By that, I mean if I go from the stock 15" up to 17s,18s or 20s, but am stuck with getting expensive, crazy low profile tires, then I end up not changing the final ratio that much and spend a load of money. So, what combination of wheel and tire would give a good bang for the buck? Thanks

The best and least expensive way to increase your wheel diameter is to go to a higher profile tire.

As an example, changing to a 205 X 70-15 or if you have the room, a 215 X 70-15.
That may be too tall of a tire at 27 inches. How wide are your steel wheels?

Do not go the 16 or 17 inch wheel route as this throws the majority of the extra weight if any out on the outer edge of the tire/wheel combination, which can actually cost you fuel economy as proven by countless people who have found that out the hard way by upgrading to the "Long Beach" factory wheels and tires.
Some reports show as much as a 5 mpg fuel economy hit.

Would adding wheel spacers to the stock 15" tires have a negative impact on fuel mileage?

Nope. They're really light and don't have any affect, at least in my experience. Make the car look a lot better, too, IMHO.