Determine actual speed vs. speed displayed on speedometer.
Whatever percentage your speedometer is off is the same percentage your odometer is off.
For example, say your speedo shows 60 MPH, actual speed is 55 and you traveled 100 miles per your odometer:
55 divided by 60 = 0.916666667
0.916666667 multiplied by 100 miles per odometer = 91.6666667 actual miles traveled.
Another way to explain the calculation using the same 60 MPH speedo reading, actual speed is 55 MPH, and odometer shows 100 miles traveled: