Quick Tire question, Come in and help
The second number (35 series) is a percentage of the first number.
You have 35% of 245mm on your front tires (sidewall height) and 35% of 285mm on your rears.
A 245/40-18 will be about the same sidewall height as a 285/35-19 because of the percentages. However because of the 1" wheel size difference, the rears would still be 1" taller than the fronts.
Also OEM C6 wheels have a very high offset and will sit inside of your fender wells quite a bit.

