Don't know why I hadn't thought of this before. Viola-tor's recent mpg measurements have been made with his 19/17 street wheels mounted and he may be comparing mpg figures obtained with his 21/18 knobby wheels. I don't know if he uses his GPS distance (which is usually a little less than an accurate front wheel odometer) or what the odometer says. At any rate the wheels do have a different outside diameter, but I don't think enough to account for the difference. Needs to be checked out though.