Even more accurate than a rollout (which I have found to be pretty rough in accuracy) is to drive your car on a favorite ride you like to bike of say 10 miles or so. Then ride your bike regularly on that ride and keep adjusting the tire circumference setting on your computer* until you get the same mileage result as your car odometer. Most car odometers are pretty accurate.


*if the 10 mile ride says 11 miles on your bike computer, set the tire circ. to 1 or 2 digits smaller, and vice versa. Remember a small circumf. adjustment will produce a big difference in the overall distance reading on a long ride.