Mich. Comp. Laws § 206.132
In the case of taxable income other than that derived from the transportation of oil or gas by pipeline, that portion of the net income of the taxpayer derived from transportation services wherever performed that the revenue miles of the taxpayer in Michigan bear to the revenue miles of the taxpayer everywhere. A revenue mile means the transportation for a consideration or 1 net ton in weight or 1 passenger the distance of 1 mile. The taxable income attributable to Michigan sources in the case of a taxpayer engaged in the transportation both of property and of individuals shall be that portion of the entire net income of the taxpayer which is equal to the average of his passenger miles and ton mile fractions, separately computed and individually weighted by the ratio of gross receipts from passenger transportation to total gross receipts from all transportation, and by the ratio of gross receipts from freight transportation to total gross receipts from all transportation, respectively. If it is shown to the satisfaction of the department that the foregoing information is not available or cannot be obtained without unreasonable expense to the taxpayer, the commissioner may use such other data which may be available and which in the opinion of the department will result in an equitable allocation of such receipts to this state.
MCL 206.132