I am thinking of upgrading my stock 16" wheels with 24.7" diameter tires to 18" with 27" diameter tires. What changes t do I need to make to ensure my speedo is calibrated correctly? Is there a tranny gear that must be swapped?

I am thinking of upgrading my stock 16" wheels with 24.7" diameter tires to 18" with 27" diameter tires. What changes t do I need to make to ensure my speedo is calibrated correctly? Is there a tranny gear that must be swapped?
A stock cts rides on 25.7" tires, not 24.7. As far as I know, the speedo cant be recalibrated. If you go to a 225/45/18, you'll only be .3" off. Thats acceptable.

Even going to 245/45R18's (which are 1" taller) work fine. The math says they're 3% off from your current setup, but my experiments with the Nav (VSS wire pulled so no speedo measurements from the car) show that the dash speedo is right on with the GPS speed with bigger tires. This could be due to the fact that the CTS-V uses these tires stock (26.7").

So by my tests, the stock Odometer would be reporting 3% higher speed/miles than reality! I never ran the test with my stock 225/50R17 tires, though, to validate this.

I wouldn't worry about it at all. Just do it!

It seems the common practice here when swithing to 18" rims to go to a 245x45x18" tire..which also happens to be the same tire used on the CTS-V.

This link works great for calculations

http://www.1010tires.com/TireSizeCalculator.asp