@khm42747 has mentioned the right size to be used in my opinion.
I have used Vitz 06 for a year, Belta 06 for 3 years and currently using Vitz 11 and Vitz 08.I tried multiple sizes and best I found was 185 65 R15 which was the best compromise among ground clearance , fuel economy, car handling and braking.
If you want the best fuel economy, go for the stock size but thats the issue, car will have ground clearance issue as JDM is supposed to be for Japan only. So it means we have to compromise on economy here. First step is easy to move from 14 inch to 15 inch tyre. It means the rim diameter has increased from 14 to 15 inch so increased ground clearance. The second step is bit tricky, say we have 195/60/R15 tyre. It means the tyre width is 195 mm and height (sidewall) is 60 percent of 195 = 117 sidewall. Theoretically wider tyre can harm your ABS calculations, traction control calculations etc (saying theoretically as I cant find out if its happening or not in my car myself) and practically "The Jali of Bumper " can be chewed by tyre or will at least create sounds on turns and high speeds (if a bit lose) by touching tyre. As we have to choose a wider tyre so go with 185/65/R15 with 120.5 mm sidewall height and less width.
My personnel opinion on fuel economy is its very relevant thing. 70 percent of time I drive alone so even with tyres r15 and heavier allow rims, my belta was giving better fuel economy then vitz 08 which was having stock sized tyres and rims but had more then 1 passenger plus driver for 80 percent times.
P.S. I might be wrong on many things above as I am no expert but happens to google and take a decision among different choices/opinions based on my interpretation.
