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You can have tiers to solve that, like vehicles under 5000lbs pay $x per kilometer driven, vehicles between 5000lbs and 10,000lbs pay $y per kilometer driven, and vehicles over 10000lbs pay $z per kilometer driven.
Wouldn’t be perfect but closer.
My preference would be to assign an equivalent single axle load to each vehicle based on make and model or avg trailer load capacity and then scale that linearly with mileage.
https://pavementinteractive.org/reference-desk/design/design-parameters/equivalent-single-axle-load/