The simplest and most accurate way to inexpensively test your odometer's accuracy is by means of a handheld global positioning satellite ("GPS") system.

These systems can be purchased at any sporting goods store for approximately $200-300.00. Activate the unit's trip odometer function and set it and your vehicle's trip odometer to zero. Drive the vehicle five to ten miles and compare the two instruments. The GPS should be accurate to within a few feet. If the reading of your vehicle's trip odometer exceeds the GPS by more than 1%, you should consider additional testing If you do not have access to a GPS unit, you can also test the accuracy of your odometer by using mile posts on interstate highways.

Starting from a convenient mile post, set your trip odometer to zero and drive ten miles. Have someone observe the mile posts as you drive to see if your odometer is keeping pace. Stop your vehicle after ten miles. If you are more than 100' from the next mile post, you should consider additional testing.

Remember, before you begin any testing, be sure that your tires are of the recommended size and that they are in good condition and properly inflated. Improper, worn and incorrectly inflated tires can produce an additional odometer error making it difficult to evaluate the instrument itself. Also remember that conclusive testing must be done by experts with the proper equipment and techniques.