[rnzo86]

hey guys just wondering how much it would affect the car if I didn't put the ideal size for the SVT but the size is for a regular focus

[sonic_mikey]

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

this will tell you everything you wanna know

[Scorcher000]

Bigger Then OE Specs

Your rotation of the tires will be less so your RPM will be lower meaning less fuel burn and your real speed will be a few KM per hour higher then your speedometer says.

Smaller Then OE Specs

Your rotation of the tires will be more so your RPM will be higher meaning more fuel burn and your real speed will be a few KM per hour less then your speedometer says.

There are many other issues that could be a cause of different tires sizes but if done properly you shouldn't have an issue.

[castone]

Do you mean just switching to a 16" instead of a 17" on your SVT?
If so no problem.
A 205-50-16 is pretty much the equivalent diameter to a 215-45-17 on an SVT.
Also a couple more options that would work for you as well
195-55-16,
205-55-16,
205-45-17,
215-40-17,
225-35-18,
215-40-18...