craftgrrl | Crochet Advice Please!

My husband has suggested that I crochet him a plush mudtire (don't ask). So in attempting this, I need to be able to create a flat circle with a hole in it. I tried doing a round of increasing every other stitch, followed by every two stitches, followed by every three stitches and it's definitely not flat.

Does anyone have any suggestions on a method of increasing that would help me make this flat?

See how it's warping? Not ideal, that's for sure. I need the big hole in the middle though. It's 18 chain stitches to start, if that makes a difference in what I should be doing.