would the lmgt4's look bad with 245-45-18( stock rear) and 275-40-18 ( aftermarket) ? just wondering my rear tires can be used for the new wheels

 

The stock rears would be to high to use in front. 240/40/18 is the correct size, and that tire has a outer diameter that is 1" less than the 245/45/18. the 245/45/18 would end up being the same height as the rear tire, and throw off the stagger and the VDC. It would be better to use the stock 225/45/18 tire, although that would induce more understeer. Also, when you have a performance car, you use performance tires. Performance tires and more unique to each other than normal passanger car tires. The performance specifications built in to one brand of tire might interfere with another. In other words, it's best to put on brand of performance tire on your new wheels in the correct sizes.

Lou