Doh! (Slapping of forehead)

A couple of weeks ago, my morning commute, which had always registered as 8.74-8.75 miles, suddenly became 9.07-9.08 miles. I thought I might have knocked the pickup, but everything seemed ok so I forgot about it. Out of the blue, I just realized that I changed from 700x40 to 700x28 tire, requiring resetting the tire size parameter. Rats! Anyway, it has been so long since I got it, I cannot find the instruction sheet on how to do this. Help! Can anyone give me a step by step, including the code for 700x28?
Thanks,
Raymond