Byron's percentages are by weight, and some of Powermaster's fuel's percentages are by weight. "The lower percentage fuels are by weight to avoid conflct with the quanity of nitro in the fuel." is the e-mail response I got from Powermaster when I asked the question. I've read that sentence a dozen times and I still don't know what it means! Neither Byron or Powermaster mention the fact on their websites that they calculate by weight. I don't know how many manufactures do calculate by weight, but I know that Morgan and Wildcat percentages are 'by volume'.