41 C.F.R. § 301-10.300 – Determining and computing mileage reimbursement.

Employees compute mileage reimbursement by multiplying the distance traveled, determined by the applicable mileage rate as follows:

Table 1 to § 301-10.300
If travel is by	The distance between origin and destination is
Privately owned automobile or privately owned motorcycle	As shown in paper or electronic standard highway mileage guides, or the actual miles driven as determined from odometer readings.
Privately owned aircraft	As determined from charts issued by the Federal Aviation Administration (FAA). Employees may include in their travel claim an explanation addressing any additional air mileage resulting from a detour necessary due to adverse weather, mechanical difficulty, or other unusual conditions. If a required deviation is such that airway mileage charts are not adequate to determine distance, employees may use the formula of flight time multiplied by cruising speed of the aircraft to determine distance. Employees must convert nautical miles to statute or regular miles when submitting a claim (1 nautical mile equals 1.15077945 statute miles).