255 vs. 235 Tire Sizes

Some members have been contemplating the upgrading of their tires from 235R22.5 to 255R22.5. The 255 tires have a larger load rating than the 235 and the rationale is that this will give a better safety margin in terms of weight-carrying capacity. Also, it is noted that the newer D's already have made the switch to the larger tires. Here are some things to take into account if you are contemplating upgrading to the larger tires.

1. Freightliner and Michelin advise that the new tire can be mounted on the original rim.
2. ALWAYS make sure that you have the same size tire on an axle. Never mix different tires sizes on the same axle.
3. The larger tire has a slightly larger rolling radius which means that your speedometer will be slightly slow. Freightliner can re-adjust your speedometer for the new tire size.
4. Your Cummins Engine Computer will be getting slightly incorrect data regarding vehicle speed. Although this can be recalibrated by Cummins, it is generally thought that this is not necessary.
5. Most importantly, Freightliner advises that the axle stops should be checked and reset when the new tires are put on to make sure that the tire does not contact any part of the suspension. This would slightly decrease the turning radius of the chassis. If a tire makes contact with the suspension, a catastrophic tire or suspension failure could result, so make sure you have it checked!