Just so you know...
Z means "over 149 MPH". How far over isn't specified, but it's less than 168 MPH.
If you wanna go 180+ MPH, you're going need "Y" (186 MPH) rated tires.
If you want to go faster than 186 MPH you'll need "(Y)" rated tires.
Edit: Oh yeah, "ZR" rated tires are good to 186 MPH and faster. (Usually)
Edit #2: Tire code break down:
We'll use the same brand but slightly different size than Jackal's recent purchase. (I couldn't find a 17" Nitto Invo tire)
NITTO Invo 245 40ZR18 93W
NITTO Invo = Make and Model
245 = Width of tire in millimeters
40 = ratio of the sidewall height to the total width of the tire (I.e., 40% of 245mm or 147mm sidewall height)
ZR = Marketing ploy. Means it might be speed rated up to 186mph or faster. (see below)
18 = Size of wheel it will fit in inches.
93 = Load rating. Means this tire can safely support 1,433lbs.
W = Speed rated up to 186mph