Right, and it's also a problem when you consider height and weight to be intertwined. Even among people of a healthy weight, a 5' tall person is going to weigh less than a 6' tall person, on average. If you really wanted to allow for pricing based on the idea that extra weight requires extra fuel consumption, but not for ascribed traits, one would have to normalize weight based on height, or instead by BMI or something that really defines the weight as "extra" and not normal for a person of a particular stature. (Unless you really do want to charge based on ascribed traits.)
Also, thank you for the "ascribed vs. achieved" nomenclature. I'd never heard that before, and I like how it avoids sounding negative and boils down the distinction very clearly.