bridgestone_passion_logo
U.S.     Canada     Français Canadien
Truck & Bus Tires


<<
Do you have a question?

The Doctor's Archives

Inflation . Tire Application . Tire Facts . Tire Maintenance
Tire Performance . Tire Specs . Tire Wear . Unidirectional Treads

Dear Tire Doctor,

Is this a true statement? "The load range or ply rating of a tire has everything to do the air pressure it can accept in order to carry more load and very little to do with the tire's ruggedness and resiliency to environmental conditions or road hazards." view reply >>

I recently purchased six Firestone T575 tires (255/70R22.5) for my Fleetwood Discovery diesel motor home. I upgraded to this size from the OEM Michelin XRV 235/80R22.5.

My question is about inflation pressures. The T575's are rated for 5,510@120 (duals 5,070@120). However my motor home is much lighter than that. My loaded axle weights are 7,840 lb. in the front (3920/tire) and 13,680 lb. on the rear duals (3420/tire).
view reply >>

I have one Bridgestone R299 295/75 22.5 load range G on my motor home. It's in the rear, mounted on Alcoa rims. The rest of my tires, including the other rear dual, are Goodyear G357s.

I'm trying to set the tire pressure, but I can't find a tire inflation table for this tire. I'm currently running 90 PSI I'm leaving on a trip tomorrow and will get another weight on the road.
view reply >>

I am in a dispute with one of our service managers.  I am trying to find the maximum pressure I can put into a 295R22.5 tire.  The service manager insists it is 100 PSI because it's on the sidewall. Can you settle this?  This question arose after a training session on suspected run flat inspections and airing the tire to 20 PSI above normal operating pressure and allowing the tire to be observed for 20 minutes as instructed in training material from the Rubber Manufacturers Association, publication dated March 1995, Volume 33, Number 2. view reply >>

Dear Tire Doctor,

Is this a true statement? "The load range or ply rating of a tire has everything to do the air pressure it can accept in order to carry more load and very little to do with the tire's ruggedness and resiliency to environmental conditions or road hazards."

Thanks, Brad

^ back to top Dear Brad,

Generally speaking, yes.

The primary purpose of a higher load range or ply rated tire is to allow for more air pressure in order to carry more weight.

In order to allow greater air pressure, there must be ‘something’ different from the lower load range tire. What is different generally is not “published” and may be the difference between tire patterns within the same manufacturer. One cannot assume the difference will increase the inherent strength of the tire, as it may be more of a compound change than a casing cable change.

In order to carry the additional load, additional air pressure must be used. A 16-ply rated tire run at 14-ply rated pressure will only carry the weight of a 14-ply tire, and in fact, due to difference in construction, may actually run hotter while doing so. On the other hand, if the additional air pressure is used and the additional weight is not carried, the 16-ply tire will be more susceptible to impact breaks and irregular wear.

If you do not need the extra load carrying capacity, you gain nothing and might actually lose by purchasing a higher load range tire than you need.

People often assume “more is better.” Some tires may be marketed or sold that way, but, in reality, if a tire manufacturer only makes a 16-ply tire and wants it used in a 14-ply application, they are really saving themselves the trouble and expense of making two products tailor-made for an application by forcing one product to cover both applications.

If I can be of any additional help, please contact me.

Best regards, Tire Doctor


Dear Tire Doctor,

I recently purchased six Firestone T575 tires (255/70R22.5) for my Fleetwood Discovery diesel motor home. I upgraded to this size from the OEM Michelin XRV 235/80R22.5.

My question is about inflation pressures. The T575's are rated for 5,510@120 (duals 5,070@120). However my motor home is much lighter than that. My loaded axle weights are 7,840 lb. in the front (3920/tire) and 13,680 lb. on the rear duals (3420/tire).

The Firestone load/inflation chart for the 255/70R22.5 tire calls for 75 PSI on the duals and 80 PSI on the singles, which allows a bit of extra weight for a margin of error. This seems ridiculously low. I believe the tires would wear better with at least 10 more pounds of pressure. Do you agree? Or should I deflate them to the pressures on the chart, even though that seems awfully low?

Thanks, Rob

^ back to top Dear Robert,

I agree with you fully.

The chart shows the maximum weight that can be carried according to selected PSI / size and wheel position. I agree that having a margin is a good idea for several reasons.

Tires will slowly loose air pressure over time (about 2 PSI per month) and this allows for air lost during the time between inflation checks.

Tires with higher pressures will allow more of a chance to pull over in the event a tire is slowly loosing air due to a road hazard before the tire reaches the critical point of not being able to carry the load.

RVs are notorious for weighing more in real life use than the weights listed on the placard.

Higher PSI normally will improve fuel mileage.

Just be careful to maintain the recommended PSI difference between the steer and drive tires, and never exceed the tires maximum cold inflation rating.

Best regards, Tire Doctor

Dear Tire Doctor,

I have one Bridgestone R299 295/75 22.5 load range G on my motor home. It's in the rear, mounted on Alcoa rims. The rest of my tires, including the other rear dual, are Goodyear G357s.

I'm trying to set the tire pressure, but I can't find a tire inflation table for this tire. I'm currently running 90 PSI I'm leaving on a trip tomorrow and will get another weight on the road.

Thanks, Dave

^ back to top Dear Dave,

The amount of air pressure in your tire depends on the amount of weight you are carrying in your RV.  Look for the chart and your tire size in the tables found under "Load and Inflation" at this site.

Regards, Tire Doctor

Dear Tire Doctor,

I am in a dispute with one of our service managers.  I am trying to find the maximum pressure I can put into a 295R22.5 tire.  The service manager insists it is 100 PSI because it's on the sidewall. Can you settle this?  This question arose after a training session on suspected run flat inspections and airing the tire to 20 PSI above normal operating pressure and allowing the tire to be observed for 20 minutes as instructed in training material from the Rubber Manufacturers Association, publication dated March 1995, Volume 33, Number 2.

Sincerely, Edward

^ back to top Dear Edward,

Thank you for the opportunity to be of assistance.

The RMA recommended practice is correct. Inflate to 120 PSI to check for casing break up (zipper).

Obviously you want to 'bleed' the PSI back to normal operating pressure before putting the tire into service on the vehicle.

The reason the recommended practice is in use is to protect against missing a potential "zipper" tire during mounting.

This practice will not damage a sound tire; in fact TRA allows a radial truck tire to be inflated up to 30 PSI (not to exceed rim capacity) above the max for use in "creep (2.0 MPH and lower)" usage.

We hope this answers your question; if not, or if you have any further questions, please feel free to contact us.

Best regards, Tire Doctor

The Doctor's Archives > Inflation . Tire Application . Tire Facts . Tire Maintenance
Tire Performance . Tire Specs . Tire Wear . Unidirectional Treads

By Popular Demand . "Ask the Doctor" Top Picks . Do you have a question?