So one of the things I use to track vehicle performance, and potential issues is mpg. If it starts to fall off or is irregular, I start looking for issues. Usually I just fill up and calculate it, compare it in my head with the last one and move on. On the ST forums everyone uses www.fuelly.com. You input your fill up info, and it tracks your mpg over the life of the car. You can also compare your mpg versus others. Pretty neat, its free, and they give you a cool signature block image.