Speed Rating Code on Bus Tires

[HB of CJ]

Just spent some frustrating time trying to find/figure out what the speed codes are for bus tires and what mph ranges are expressed.  No go.  Does anyone make "high speed" rated truck/bus/emergency vehicle tires?   How are they rated and is the rating code somewhere on the tire?   Thanks. 

[TomC]

Usually if the tire is rated less than 75mph, it will be marked on the tire.  There are no speed designations on truck tires like there are on cars.  What size tire do you have?  Good Luck, TomC

[RJ]

HB -

Agree with Tom on this one.  Most highway coach tires do not have any speed designations on them, but transit tires often do, frequently maxed at 50 or 55 mph.  Even then, the manufacturers often recommend a "cooling off" period if driven for extended periods at highway speeds.  (Usually 30 minutes every 200 miles.)

TTBOMK, transit tires differ from highway tires in several ways.  First, they have much thicker sidewalls, as operators of these buses in revenue service tend to think that curbs are part of the braking system, so they're scrubbed against them often.  These thicker sidewalls generate a lot more heat at freeway speeds than does a tire designed for the highway.

Second, transit brakes tend to run hotter than highway coaches (since they're often stopping every block w/o a jake), so the rubber compound in the tire must be able to tolerate the heat load induced through the wheel, in addition to the heat induced from normal flexing.  This is particularly true in the bead area.

BTW, the most common bus tire size today, for revenue service, is 12R22.5, which crosses over to the metric-sized 315/80R22.5.  In the past, 10.00 x 20 and 11.00x20 were common tube-style sizes, and a lot of conversions run either 11R22.5 or 11R24.5s.  Won't be long before all we see are metric-sized tires, as the industry is slowly moving in that direction.

FWIW & HTH. . .

[HB of CJ]

Thanks Tom and Russ.  The Crown has 12RX22.5's...new Firestones on the front and a mix and match of four (4) different caps on the back including  Bridgestone, Michilen, Goodyear and Firestones.  Guess when the schoolie was surplused out, they mounted what they had.

[TomC]

You can run the 12R's, which will be enough rating for you (7390lb front and 27,120lb rear- Michelin XZE). Or you could go with the more modern metric which is  315/80R22.5 (9090lb front and 33080lb rear-Michelin XZA1).  They are almost identical in size- 12R is 19.8 radius with 486rpm and the 315 is 19.6 radius with 489rpm.  Both are 75mph rated tires.  I would suggest the 315's, since the 12R is harder to find and will continue to get harder.  Good Luck, TomC