When I look at my G8 from the side it always looks like the rear end is sitting higher than the front. I was thinking about getting new lowering springs on the rear only. I really want the 20" wheels I have to fill up the tirewell but they don't right now. I put a set of Pedder 1/2" drop springs on my GTO rear end only with 19" wheels and everyone thought it looked great. What Pedder spring would I want to use if I did this, only looking for about a 1/2" drop or so? Am I crazy to not do the front springs too? All the Pedder springs I see on line just refer to xxx millimeters with 19" tires, xxx millimeter with 18" tires. I don't find any place that just says 0mm drop, 1/2" drop, etc.

I can't answer your question but don't forget that the front wheel well is bigger than the rear.

The rear end is higher. I don't think there are zero drop springs for the G8. My pedders suspension put my rear drop at 20mm. The front was 10.
I had a sheet with my ride heights but I can't seem to find it.

I think Ray is correct, the openings are different the fronts bulge out and the rake is really in the body. Look the the body lines below the doors. Pretty even, I think the measurements Ray is talking about are around 650 mm measured from the lower lip on the wheel to the bottom of the fender well. Mine are within 1mm all around.

The rear end is higher. I don't think there are zero drop springs for the G8. My pedders suspension put my rear drop at 20mm. The front was 10.
I had a sheet with my ride heights but I can't seem to find it.

What spring did you use? I think I saw that you installed the Track II but I didn't know what spring that was. I didn't want to spend the money to replace the shocks/dampers as well and thought just the springs would lower it down. I'm going to throw some sand bags in the trunk and see if I can get it down 20mm to see what that would look like. The wheel wells look good in the front, it's just the back that seems to look weird to me. I don't care that much about handling but don't want it to be all squirrelly either.

Check out the Street II. However, I used the rear springs from the Track II.
The part numbers are listed on Pedders USA web site.
So...basically I have a Track II without the sway bars. Maybe a couple more bushings.