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You must measure actual miles driven somehow -- the easiest way is to drive on a highway that has markers every 10th of a mile, and reset the Trip (B?) odometer at an even mile marker. Drive for at least 20 miles (as far as you can) and compare the Trip odometer to the measured distance. Add (or subtract) a percentage to the Trip odometer to get you close to the measured distance; erring on the conservative side.
For example, if you drove 25 measured miles, and the trip odometer only read 23.8 miles, you need to add 5% to get to 24.99 miles.
Another way to measure the actual distance you drive is use a GPS, though this sometimes doesn't account for elevation changes and might be off a bit. A third way to measure the actual distance is to use Google maps for directions to go between 2 points, and then drive that specific route, and measure it on the trip odometer.
You cannot depend on specs of the tires as the tire may be worn, or you may be running them at a higher pressure, etc.
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