Jonathan does a great job explaining how to model a tire in this video on blendercookie.com. However, I came across a problem where some of the elements were getting distorted in my tire and this was preventing the ends from merging. Here’s what I mean:
The distortions that you see in the first image are because of the resolution for the guiding circle. The default value is 12 which gives lots of distortions. By changing it to 64 (max), all distortions disappeared.