Okla. Stat. tit. 69 § 1729
In determining the amount of motor fuel consumed on the Oklahoma turnpike projects each month, the Oklahoma Tax Commission shall divide the total miles traveled on the turnpikes by passenger automobiles, single unit trucks, and combination trucks and buses as certified each month by the Authority, by the average number of miles per gallon of motor fuel consumed by each of those classes of motor vehicles. For the purpose of this section and the following section, the Oklahoma Tax Commission shall use the following rates for motor fuel consumed on the Oklahoma Turnpikes:
Passenger Automobiles ................15 miles per gallon
Single Unit Trucks ...................10 miles per gallon
Combination Trucks and Buses .........5 miles per gallon
Okla. Stat. tit. 69, § 1729