Under inflated or deflated tires are known to cause increased forward drag and lateral steering effects on vehicles. These effects are commonly suggested to be the cause of driver loss of control and subsequent vehicular accidents. The increased drag and steering effects of under inflated or deflated tires are frequently an issue in an accident reconstruction. This paper documents the results of a series of tests conducted to determine the magnitude and effects of under inflated or deflated tires on cars and light trucks. The test also establishes a method of testing that can be used to determine steering effects for other vehicles and speed conditions. Six vehicles ranging from a compact passenger car to a 3/4 ton pickup truck were tested. The test methodology was simple and produced repeatable test results up to the 45 mph speed defined as a limit for the tests.The first phase of the tests involved the measurement of forward deceleration rates during a series of coast to stop tests with all of the vehicle tires initially inflated to the rated pressure. This was followed by tests at various under inflation pressures. The deceleration rates versus inflation pressures are documented and discussed. As anticipated all vehicles experienced an increase in the coast to stop deceleration rates at under inflated tire pressure conditions. However, the increase in the deceleration rates were not dramatic in magnitude. When all four tires on the test vehicles were deflated to 10 psi, the coast to stop deceleration rates still represented relatively small deceleration profiles.The second phase of the tests was performed with a totally deflated tire on either the front or rear axle. The forward deceleration and lateral acceleration rates, and the corresponding dimensional lateral motion profiles were measured during the coast to stop tests with the deflated tires. The forward and lateral acceleration rates and the lateral offset travel distances are documented and discussed. During the test, the test driver was always able to easily maintain control of the test vehicles and steer them in either a straight path or curved path with either a deflated front or rear tire. The driver was always able to easily regain control and steer the test vehicles after initially releasing the steering wheel and allowing the vehicle to initiate lateral motion as a result of the deflated tires. This suggested that a blow-out single tire event can be a controllable condition.These tests provided consistent results within the test speed ranges up to 45 mph. The results may not be similar at higher speeds and the results should not be randomly extrapolated to higher vehicle speeds without a valid basis. If higher speed affects are desired, the same test methology is considered to be a reasonable approach to gather data.