255/40/17 same height as the stock 245's

No. Second number is ratio of height to width. If your talking stock rims, 275 40s are the same OD as stock 245/45s.

See this site:

http://www.miata.net/garage/tirecalc.html

Also tire sizes vary by manufacturer, not all 255/40/17 are the same size.

You can find the tire you want and search for it on tire rack. Then click specs and it will tell you the O.D.

The 245/45/17 yokohama has an O.D. of 25.6
The 255/40/17 yokohama has an O.D. of 25.1
The 245/45/17 stock goodyear has an O.D. of 25.7
The 255/40/17 BFG T/A has an O.D. of 25.0

ETC. ETC.

My wife has 255/40/17 yokohama avs es100's on her GT and loves them by the way.

Here are the specs for the yokohama.
http://www.tirerack.com/tires/Spec....WR7ES100&vehicleSearch=false&fromCompare1=yes