S.D. Codified Laws § 10-47B-176
The tax liability of an interstate fuel user licensee shall be determined based on average fuel consumption for all qualified vehicles operated by the licensee within this state. Average fuel consumption is equal to the total fleet vehicle miles traveled divided by total fleet gallons of fuel consumed to calculate an average mile per gallon (AMPG) for all vehicles operated. This AMPG is then divided into the total miles traveled within South Dakota by all vehicles to determine the total gallons of fuel attributable to South Dakota operations. The gallons thus calculated shall be multiplied times the motor fuel, ethyl alcohol, methyl alcohol, biodiesel, biodiesel blend, liquid natural gas, liquid petroleum gas, compressed natural gas, special fuel, or any combination thereof, per gallon tax rate currently in force at the time the return is completed to determine the total tax due from the licensee. This tax liability may be credited in the amount of any South Dakota tax on motor fuel, ethyl alcohol, methyl alcohol, biodiesel, biodiesel blend, liquid natural gas, liquid petroleum gas, compressed natural gas, special fuel, or any combination thereof, paid at the time the fuel is purchased within this state. Over purchases of motor fuel, ethyl alcohol, methyl alcohol, biodiesel, biodiesel blend, liquid natural gas, liquid petroleum gas, compressed natural gas, special fuel, or any combination thereof, which result in a tax overpayment must be refunded to the licensee in accordance with the provisions of § 10-47B-177.
SDCL 10-47B-176