use this to get an exact difference in size:
http://www.1010tires.com/tiresizecalculator.aspof if you need more of a visual (takes longer to load and can't compare as many at once)
http://www.miata.net/garage/tirecalc.htmlCan I ask why you feel you need more sidewall? The only reason sidewall does do is allow is more deflection to allow running a lower pressure in the tires. But if you are looking for more comfortable ride and you are comparing a stock tire to a 40 series 17 inch tire then yes you will notice the difference. but this has to do with more than just tire sidewall. it also has to do with the type of sidewall, weight of the rim, etc.
If you go from a reinforced performance tire to a touring tire the side wall of the tire becomes "softer" also to allow for a more comfortable ride. Only on an extreme sidewall size change will you notice a difference in ride, like that stock 60 series to a 40 series. But the side wall of a 205/40r17 is @ 3 1/4 inches and the side wall of a 185/60r14 is 4 3/8 inches. that is only an 1 1/8 inch difference for 3 more inches of rim.