Calibrating your Odometer
Even in identical cars, the tires would have to have the same pressure and the same tread depth and loading to make the odometers read the same.
For this reason, each rally starts with an odometer calibration run. Normally this section has a specified time in which to reach the end or it may be separate from the timed runs all together in the case of a rally race.
In any case, the basic procedure is the same. You drive the odometer calibration route making note of your mileages at the points referenced in the route instructions.
Special note on using trip odometers: If you are using the mechanical in dash trip meter, there is often some lag before it starts to count after it has been reset. You will have to learn how yours operates if you want to reset it at each zero point. Usually there is about 2/100ths lag in a stock Odo after the reset button has been pushed. Many rally masters do not know this and think they are starting at zero when they are not. If you are running a computer pay careful attention to the early readings to make sure you both started at the same point. In most cases, my navigators do not use the first "zero" points unless they know the rally master has done things right.
So after you get your numbers collected, check to see if they are consistent. Do you get close to the same factor at each data point? If you just use the end point you could fall victim to a reading error - either your own or the rally master's.
Follow your computers instruction for calculation of the correction factor. In the compurally software, you just key in the mileages and it does the work for you.
If running the stock odo, divide the distance you measured by the rally masters mileage. Then use this number to multiply by the official mileage to know what your odo should read at that point.