Do you have to have your speedo recalibrated when you get 18's? I am looking at Z06 18X10.5 for the back and 17X9.5 in the front. Also would you have to recalibrate for 18's all around. I still plan on running 275/40ZR17 and 18's all around

 

You only need to "correct" the speedometer if you change the outside diameter of the tire. When you go to a larger diameter wheel, you generally use a tire with a lower section height (aka - "aspect ratio")..... the larger wheel diameter is offset by the thinner sidewall dimension. The key is to match the outside diameter of the tires that came on your car from the factory.

275/40-18 would cause an error of about 4%, since it is about 1" larger in diameter than the original tires. You would be better off with a "35-series" tire... e.g. - 275/35-18 or 285/35-18.

If your car came with 245/50-16 tires, you can use the following with little (less than 1% error) or no speedometer/odometer error:

245/50-16
275/40-17
285/40-17
315/35-17
275/35-18
285/35-18
315/30-18
275/30-19
285/30-19

Speedometer calibration is determined by how fast the driveshaft rotates, since the sensor is on the transmission output shaft. Front tire diameter will not affect the speedometer on a RWD 4th Gen F-Body. You do want to keep the front tires close to factory diameter as well, though.