Tom!, please accept my experience. Until I get a BMI, I always checked tweak, and it did move around from time to time. With the BMI, my experience is that it doesn't move around.
However sceptical you are about that, you will know that unless you have personally experienced something, you can't understand it - so it was for me. Tweak works as Jason says, but with his car it is part of a forgotten (almost!) 12th experience!!
