Issue link: http://read.dmtmag.com/i/41477
Appendix CHANGES IN TOP SPEED WHEN TIRE REVOLUTIONS PER MILE CHANGES GEAR RATIO A change in tire dimension will result in a change in engine RPM at a set cruise speed* that will result in a change in speed and fuel economy. The effect of tire size change on gear ratio should be considered in individual operations. A decrease in tire radius will increase tractive torque and increase indicated top speed. An increase in tire radius will reduce tractive torque and decrease indicated speed. Tire Revs./Mile – Speed– Size: These factors can affect engine RPM if corresponding changes are not made to engine ratios. Example: Going from larger diameter tire to smaller diameter tire. If you currently run a 275/80R22.5 MICHELIN XDN® 2 tire (511 Tire Revs./Mile) and change to a 445/50R22.5 MICHELIN X One® XDN® 2 tire (515 Tire Revs./Mile), the speedometer will indicate a slightly higher speed than the actual speed the vehicle is traveling. Final Tire Revs./Mile – Initial Tire Revs./Mile = Initial Tire Revs./Mile 515 - 511 = 0.0078 or .78% (< 1% change) 511 So when your actual speed is 60 mph, your speedometer will read 60.47 mph. MICHELIN X One Tire Size 445/50R22.5 Dual Size 275/80R22.5 MICHELIN X One Tire Tire Revs./Mile 515 (X One XDN2) Dual Tire Revs./Mile 511 (XDN2) MICHELIN X One Tire Size 455/55R22.5 Dual Size 11R22.5 or 275/80R24.5 MICHELIN X One Tire Tire Revs./Mile 492 (X One XDN2) Dual Tire Revs./Mile 496 (XDN2) Rule of Thumb: When going from a lower Tire Revs./Mile to a higher Tire Revs./Mile, the actual vehicle speed is less than the speedometer reading. When going from a higher Tire Revs./Mile to a lower Tire Revs./Mile, the actual vehicle speed is greater than the speedometer reading. * Exceeding the legal speed limit is neither recommended nor endorsed. 92