RE: My Ace hardware now carries Stihl oil.
All these ratios are based on fluid ounces which is actually a volume measurement and not a actual weight measurement.
A gallon of gasoline is 128 fluid ounces but if one of you go to measure what it actually weighs you will find the net weight to actually be close to 96 ounces.
So to divide (2 X 128) by 6.4 you get 40. That means 40:1
or to divide (2.5 X 128) by 6.4 you get 50. That means 50:1