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TIRE REVOLUTIONS PER MILE CALCULATION MEASURED TIRE REVOLUTIONS PER MILE At Michelin, Tire Revolutions Per Mile (Tire Revs./Mile) are officially determined using the SAE (Society of Automotive Engineers) Recommended Practice J1025. The test tires are placed as singles on the drive axle of the test vehicle and loaded to the maximum dual load rating of the tire and set to the corresponding pressure. The vehicle is then driven over a straight 2-mile section at 45 mph while the number of revolutions are counted. (Since speed minimally affects the results for radial tires, other speeds are allowed.) Averaging four runs that are within 1% of each other then derives the Tire Revs./Mile measurement. Afterwards, the results are double-checked using shorter distances that are more easily obtained. In addition to these, the test tire is compared to a known baseline tire on a road wheel. This latter method is very accurate and very repeatable when using a similar baseline tire with a known Tire Revs./Mile. The SAE procedure recognizes that within the test method itself there will be some variation. In fact, there are other factors that cause variation on Tire Revs./Miles among similar tires. Be aware that just because similar tires have the same overall diameter this does not necessarily mean that they will have the same Tire Revs./Mile. The SAE procedure determines the Tire Revs./Mile to within ± 1.5%. Some factors, which cause variation among tires, are: • Load and Pressure – A difference in Load/Pressure could alter the Tire Revs./Mile measurement by as much as 1.5%. If pressure is constant, going from an empty vehicle to a fully loaded vehicle can change the Tire Revs./Mile by 1 to 1.5%. • Treadwear – The Tire Revs./Mile varies from a new tire to a fully worn tire. This can affect Tire Revs./Mile by as much as 3% from the rated Tire Revs./Mile. • Tread Geometry – The height and stiffness of the blocks and the shape of the tread pattern can affect Tire Revs./Mile. • Torque – The presence of driving and braking torque can affect the Tire Revs./Mile. • Type and Condition of Pavement – Asphalt vs. concrete, wet vs. dry can create difference in Tire Revs./Mile. CALCULATED TIRE REVOLUTIONS PER MILE Michelin Equation: Tire Revs./Mile = 20,168 / (O.D. - .8d) O.D. = Overall Diameter d = Correction for deflection d = (O.D./2) - SLR SLR = Static Loaded Radius (Ref. Data Book) Example: 275/80R22.5 MICHELIN XDA ENERGY ® ® New Tire O.D. = 40.5 SLR = 18.8 d = (40.5/2) - 18.8 d = 1.45 Tire Revs./Mile = 20,168 / (40.5 - (.8 x 1.45)) = 20,168 / (40.5 - 1.16) = 20,168 / 39.34 Tire Revs./Mile = 512.6 (Calculated) Vs Data Book (Measured) Tire Revs./Mile = 513 At 50% Worn O.D. = 40.1 SLR = 18.6 (13⁄32nd used is approximately a 0.2 inch reduction of SLR) d = (40.1⁄ 2) - 18.6 d = 1.45 Tire Revs./Mile = 20,168 / (40.1 - (.8 x 1.45)) Tire Revs./Mile = 518 (Calculated) Section Nine: Appendix 131