I'm a idiot.

So I just decided to do some research on my car 350z. I'm running 245/40/18 and 275/40/18 on 18 x 7.5 rims. Just how dangerous is this? I've had this setup for about 3 years...

Should I go ahead and grab a wider rim and tire?

To figure out tire sizes you need to know a few things. On a 245/40/18 245 is the width of the tire in millimeters. 40 is sidewall height percentage of the width. 18 is obviously the rim diameter.

You need to figure out the overall height of both the front and rear.

245/40/18

245/25.4 = 9.65" wide (25.4 millimeters in an inch)

9.65 x .40 = 3.86"

(3.86 x 2) + 18 = 25.75" tall (You have to multiply the sidewall height by two because that is only one side of the sidewall)

Doing the same math the 275/40/18 is 26.66" tall and 10.83" wide.

Assuming you want to keep the same/similar height, both front and rear, you would want to go with a 275/35/18. It comes in at 25.58" tall.

I would not run a 275 on a 7.5" wheel. You need at least 8.5", if not 9" In my opinion the 245 is the limit I'd ever go on a 7.5" wheel, but again, I'd probably want at least 8.5".

Here's a link to an online tire store.
Notice the relationship between the TREAD width of the tire to the recommended rim width.
http://www.tirerack.com/tires/tires.jsp?tireMake=Michelin&tireModel=Pilot+Super+Sport&partnum=835YR8PSSXL&vehicleSearch=false&fromCompare1=yes&tab=Specs