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The Doctor's Archives

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

Dear Tire Doctor,

I noticed that the dual tire weights are always lower than single tire weights.  Is it due to friction heat from the other tire? Load divided by two? Or another reason? view reply >>


My company is considering buying a trailer with Firestone T575 255/70R22.5 tires to haul a fairly heavy vehicle at low speeds. We need to know the low-speed (40 km/h or less) load limit of these tires, and if they are sufficient to handle the load. view reply >>


Please explain the differences between 22.5 low profile and 24.5 low-profile tires. Does one get better wear over the other? Does one get better fuel economy over the over? view reply >>


I need to find the RPM on the Bridgestone M720 295/75R22.5. I need to set the ECM on our trucks. view reply >>


How can you tell how old a Bridgestone tire is when it comes from Japan? Is it the 4-digit number or the 3-digit letters? view reply >>


I am doing an independent review/comparison of tires for a client and would like to get the undertread thickness of the R192 transit tire in the size 275/70R22.5. view reply >>


What size drive axle tires do you recommend for my 1996 Volvo tandem axle tractor?  It's used 100 percent highway miles only.  The load weight ranges from 12,000 to 41,000 pounds. I operate primarily on the East Coast from Florida to Maine. view reply >>


What is difference in height of an 11R22.5G tire
vs. 255/80R22.5G ?
view reply >>


I have a truck with a GVWR of 66,000 lbs with a wheelbase of 302" and a tandem rear axle spacing of 60".  The rear tires are Bridgestone M711 11R22.5 G.  What is the length of the contact patch of the tires? view reply >>


How do we calculate a tire's overall diameter? For example, using 11/22.5 and 295/75/22.5, I multiplied the two sides of each tire and they do not compute. (11 x 2 = 22 x 2 = 44 inches)  Regarding the 295/75/22.5 size, it doesn't match the dimensions on your web site, especially when I change to low profile tires.
view reply >>


Our truck uses 24.5 tires all the way around. Can we change the steers to 22.5 with the smaller ribs but leave the drives at 24.5? We were told this would affect the frame rake. We would welcome your comments or additional questions on this matter. view reply >>


The 385/65R22.5 and 425/65R22.5 M844 is speed rated at
65 mph but the R194 is 75. Why?
view reply >>


I found a set of Firestone steer tires with the number 589 at an auction.
I've never heard of this model; can you tell me more?
view reply >>


Is there a formula to determine what the maximum weight an
1100 x 20 load range H radial tire can handle (single) with a
maximum speed of 5 mph. Sidewall states (Single 7390 @ 120 PSI cold)
(Dual 6780@ 120 PSI cold)
view reply >>

Dear Tire Doctor,

I noticed that the dual tire weights are always lower than single tire weights. Is it due to friction heat from the other tire? Load divided by two? Or another reason?

Thanks. Lena
^ back to top Dear Lena,

The main reason that dual tire load limits are set at a lower rating than single tires is that tires paired up as duals do not always contact the road surface equally. Examples of this would be ruts in the road surface, climbing over curbs, mismatches in inflation pressure and/or remaining tread depth, to name a few.

This unequal road surface contact results in one of the dual tires ending up carrying more than its fair share of the load. In order to provide a “safety cushion” for when this happens, the maximum weight allowed is set lower than for a single application.

Best regards, Tire Doctor


Dear Tire Doctor,

My company is considering buying a trailer with Firestone T575 255/70R22.5 tires to haul a fairly heavy vehicle at low speeds. We need to know the low-speed (40 km/h or less) load limit of these tires, and if they are sufficient to handle the load.

Thanks Richard
^ back to top Dear Richard,

Thank you for the opportunity to be of assistance. The normal load capacity for the 255/70R22.5 H T575 in a dual position is 2,300 KG @ 830 KPA (5,070 lb. @ 120 PSI). This allows the tire to be run at up to 120 km/h (75 mph).

TRA allows a tire run at a maximum of 40 km/h (25 mph) to add 12% to its load capacity by adding 69 KPA (10 PSI) inflation. This would allow the tire to carry 2,576 KG @ 899 KPA (5,678 lb. @ 130 PSI).

However, the maximum load and inflation pressures of the rim must not be exceeded, so you will have to confirm that the wheels used are approved for that load/inflation.

I hope this answers your question to your satisfaction. If not, please feel free to contact us.

Regards, Tire Doctor

Dear Tire Doctor,

Please explain the differences between 22.5 low profile and 24.5 low-profile tires. Does one get better wear over the other? Does one get better fuel economy over the over?

Thanks, Scott
^ back to top Dear Scott,

Thank you for this opportunity to be of assistance.

Many large fleets traditionally operate low profile (LP) 22.5" tires. Prior to January 2005, the Federal Excise Tax, or FET, was based on a tire's weight, and since 22.5" tires weigh less, the FET was lower, thus making them the popular choice. After the FET law changed, most of these fleets continued to operate the 22.5" tires to avoid duplicating inventories.

There are several small advantages for each size.

LP24.5" tires, according to tests performed to SAE standards by an independent testing laboratory, tend to have a slightly lower rolling resistance at a given weight and speed than do LP22.5" tires, and thus have the potential of a very slight fuel mileage advantage.

LP24.5" tires turn fewer RPM than LP22.5" tires and are credited with being able to deliver slightly higher removal mileages, although this may be disputed by some.

LP22.5" tires weigh less, which allow a slight increase in maximum payload.

LP22.5" tires have a lower center of gravity, promoting slightly better handling characteristics.

LP22.5" tires typically feature the latest technology.

LP22.5" tires are far more popular, which can influence availability and casing pricing.
Generally speaking, the differences are so slight, that it would not pay to change a current fleet from one size to the other due to duplicate inventory costs and problems.

Best regards, Tire Doctor

Dear Tire Doctor,

I need to find the RPM on the Bridgestone M720 295/75R22.5. I need to set the ECM on our trucks.

Regards, Hector
^ back to top Dear Hector,

The RPM is 512. Tire specs are always available on our web site. Go to the Truck Tires button, select Tire Selector and choose the M720.

Regards, Tire Doctor

Dear Tire Doctor,

How can you tell how old a Bridgestone tire is when it comes from Japan? Is it the 4-digit number or the 3-digit letters?

Regards, Jason
^ back to top Dear Jason,

Prior to the year 2000, the last 3 numbers in a DOT were the date code.  For example, 129 would indicate the 12th week of 99.  After 2000, the system went to 4 digits, like 1201, which would be the 12th week of 2001.

Hope this helps.  If you have other questions, just contact our technical services office at 1-800-847-3272.

Regards, Tire Doctor

Dear Tire Doctor,

I am doing an independent review/comparison of tires for a client and would like to get the undertread thickness of the R192 transit tire in the size 275/70R22.5.

Thank you, Gregory

^ back to top Dear Gregory,

Thank you for the opportunity to be of assistance. In response to your question concerning "undertread" thickness on the 275/70R22.5 R192 transit tire, the answer is 6/32nds on both the shoulder and centerline.

Best regards, Tire Doctor

Dear Tire Doctor,

What size drive axle tires do you recommend for my 1996 Volvo tandem axle tractor?  It's used 100 percent highway miles only.  The load weight ranges from 12,000 to 41,000 pounds. I operate primarily on the East Coast from Florida to Maine.

Regards, Richard

^ back to top Dear Richard,

Your vehicle was designed with a specific tire size in mind.  For instance, the speedometer is calibrated based on the size of your tires.  Changing to another size is done with the assistance of the OEM.

Standard size tires, 11R22.5 and 11R24.5 tires will carry more load than the low profile 22.5 and 24.5 sizes.  For example, our 11R24.5 M726 EL drive tire in a dual set up will carry 150 lb. more than the 295/75R22.5 and 285/75R24.5 sizes.

Additional things to consider are casing availability and value.  295/75R22.5 is the most popular size tire on the market, so naturally they are the most plentiful and valuable.  Other benefits of low profile tires include reduced weight, better handling and lower costs.

We hope this has been of some use in answering your question.

Regards, Tire Doctor

Dear Tire Doctor,

What is difference in height of an 11R22.5G tire vs. 255/80R22.5G ?

Regards, Jim

^ back to top

Dear Jim,

The Overall Diameter difference between the two sizes in your inquiry is approximately 2.8" (it will vary slightly between tire patterns).

11R22.5 G ~ 41.3"

255/80R22.5 G ~ 38.5"

The Loaded Radius difference - the distance from the road surface to the axle center that determines ride height - is approximately 1.3".

11R22.5 G ~ 19.2"

255/80R22.5 G ~ 17.9"

The difference in diameter between the two tires results in a difference in Revolutions Per Mile of ~ 37 RPM

11R22.5 G ~ 501 RPM

255/80R22.5 G ~ 538 RPM

Another bit of advice: the size 255/80R22.5 G is a "European standards" size, and is not common to all tire manufacturers.

Tire size 265/75R22.5 G is a direct replacement size and may prove easier to find from various tire manufacturers in the North American market.

Best regards, Tire Doctor


Dear Tire Doctor,

I have a truck with a GVWR of 66,000 lbs with a wheelbase of 302" and a tandem rear axle spacing of 60".  The rear tires are Bridgestone M711 11R22.5 G.  What is the length of the contact patch of the tires? 

Best regards, Leanne

^ back to top Dear Leannne,

Thank you for contacting Bridgestone for help with your question.

If we assume the drive tandems are being run at the maximum legal (U.S.) weight of 34,000 lb., we would have a load of 4,250 lb. per tire (34,000/8). Let us also assume an inflation pressure of 95 PSI.

Using the T&RA 'Pavement Contact Pressure Guide' formula, we get a gross contact area of 58.4 square inches. Since the tread width of the 11R22.5 M711 is 8.1 inches, we divide 58.4 by 8.1 and arrive at the footprint length of 7.21 inches.



We hope this has been of some use in answering your question.

Best regards, Tire Doctor

Dear Tire Doctor

How do we calculate a tire's overall diameter?
For example, using 11/22.5 and 295/75/22.5, I multiplied the two sides of each tire and they do not compute. (11 x 2 = 22 x 2 = 44 inches)  Regarding the 295/75/22.5 size, it doesn't match the dimensions on your web site, especially when I change to low profile tires.

Regards, Vida

^ back to top Dear Vida,

Thank you for your question.

First off, let us say that tire sizing / dimensions are not as simple as they might appear.

In fact, if you look through the data pages on the web site for the various tire patterns, you will notice that tires of the same size, but different pattern do not always have the same diameter.

This is due to the fact that the tires need to be of approximately the same diameter when buffed for retreading.

Since different tire patterns often begin life with different original tread depths, this results in new tires having varying original diameters from pattern to pattern.

For example, a new R250 steer tire, which starts with 19/32nds of original tread depth, has an original diameter of 41.3" in size 11R22.5, while a new M726 EL drive tire, with an original tread depth of 32/32nds, has an original diameter of 42.1" in the same size.

This allows the tires to end up with the same basic diameter when worn out. This is why it is best to refer to the data pages for exact original diameter.

Now, to answer your question on computing the "approximate" diameter of a tire by its size: First, we must understand there are different nomenclatures used for stating tire size.

For example: 11R22.5, an older format of expressing tires size, means a tire width of 11 inches and use on a 22.5" wheel. This nomenclature is used for what is called "90" series, meaning that the tire's sidewall height is 90% of the tires overall width.

To compute the approximate size of an 11R22.5:  Tire sidewall height = 11 inches (tire width) X 90% (tire profile) X 2 (two sidewall sections, one above the wheel, one below) + 22.5 inches (wheel diameter)
11 X .90 = 9.9" X 2 = 19.8" + 22.5" = 42.3"
In low profile tires, a more modern metric sizing is used: i.e. 295/75R22.5. Here, the width (295) is expressed in millimeters, the profile (75) is included in the size, and the wheel is still shown in inches (22.5) To compute the approximate size of a 295/75R22.5: Convert the metric width to inches (295 divided by 25.4 = 11.6 inches) then compute the same as above - only use a profile of 75%
11.6 X .75 = 8.7" X 2 = 17.4" + 22.5" = 39.9"
We hope this answers your question.

Best regards, Tire Doctor

Dear Tire Doctor,

Our truck uses 24.5 tires all the way around. Can we change the steers to 22.5 with the smaller ribs but leave the drives at 24.5? We were told this would affect the frame rake. We would welcome your comments or additional questions on this matter. 

Thanks, Gary

^ back to top Dear Gary,

Thank you for the opportunity to be of assistance.

Yes, changing the front axle from 24.5 to 22.5 wheels/tires can potentially affect the frame angle of the tractor.

You did not specify if you were running 90 series or low profile tires, so we'll address both in the following examples.

If you are currently running 11R24.5:
A standard 11R24.5 steer tire has a static loaded radius (distance from road to axle center when loaded) of 20.2 inches.

The same tire in size 11R22.5 has a static loaded radius of 19.2 inches.

Thus you have effectively lowered the nose of the tractor by (one) inch.

If you are currently running low profile 24.5" 

A standard 285/75R24.5 low profile steer tire has a static loaded radius of 19.4 inches

The same tire in size 295/75R22.5 low profile has a static loaded radius of 18.8 inches

Thus you have effectively lowered the nose of the tractor by (.6) inches

However: if you are currently running 285/75R24.5 drives (19.4" static loaded radius) you could conceivably run 11R22.5 steer tires (19.2" static load radius) with no significant difference in height.

The main concern we would have with mounting 22.5 steer and 24.5 drive is the dual inventory for spares/replacements and the chances of mistakenly mixing sizes on the same axle.

We are also concerned with shifting additional weight to the steer axle. Changing the frame angle reduces the effective caster angle of the alignment. However, these items could possibly be dealt with by adjusting the ride height of the air ride suspension.

The only time this (22.5 steer, 24.5 drive) would normally be done is in construction type vehicles where they run 11R24.5" drive tires, and 425/65R22.5 wide base tires on the steer.

Best regards, Tire Doctor

Dear Tire Doctor,

The 385/65R22.5 and 425/65R22.5 M844 is speed rated at 65 mph but the R194 is 75. Why?

All the best, Bill

^ back to top Dear Bill,

The maximum speed of a tire is basically determined by a formula of heat vs. load value.

A tire's operating temperature is determined by several factors, including speeds, load/inflation, tire design/construction, any changes in one of these factors could affect the tire's heat to load value. Additionally, a change in any of these factors could require and/or allow a change in one or more of the other factors.

Now consider that the M844 is designed for on/off highway use while the R194 is designed for on-highway use.

The M844 original tread depth is 23/32nds, while the R194 wide base has just 16/32nds when new. The deeper the tire's tread depth, the more heat is generated and held within the tire as it cycles through its footprint.

However, the biggest difference between the two tires is the type of rubber compound used. The M844's compound is designed to resist cuts and tears because it's designed to be used in on/off highway applications. The R194, which is designed to operate on-highway, uses a cooler-running rubber compound.

Since the M844 requires a deeper tread depth and tougher rubber compound to do its job properly, and the load/inflation is a given, the only way to compensate for the extra heat is to limit its speed to stay within the acceptable tire heat/ load range.

Best regards, Tire Doctor

Dear Tire Doctor,

I found a set of Firestone steer tires with the number 589 at an auction. I've never heard of this model; can you tell me more?

Sincerely, Glen

^ back to top Dear Glen,

Thank you for the opportunity to be of assistance.

The Firestone "589" is a tire that was produced years ago for steer tire use. Production of the "589" ended in 1997, so the tires that you observed would by necessity be a minimum of 8 years old.

Their exact age could be determined by the last 3 digits of the DOT number - with the last number indicating the year produced, and the 2 numbers preceding it telling the week of the year produced.

For example:

If the last three digits of the DOT number were 446

The 6 tells us the tires were produced in 1996

The 44 tells us they were produced the 44th week

of that year
Needless to say, the tires are quite old. And they would not be covered by any warranty.

Tires do deteriorate with age, even if not in use.

We warranty tires for 6 years from date of manufacture. This is based upon our belief that a tire should be replaced based on age, even if it is not worn out.

It is also very common in the industry for major fleets to limit the age of a casing that they will allow to be retreaded. Many fleets use 5 or 6 years as their maximum, the maximum allowed casing age of any major fleet that I am familiar with is 8 years, and then only for tires limited strictly to trailer use.

Based on these principles, we would not be able to recommend that the tires you observed would be a 'recommended' purchase.

Best regards, Tire Doctor

Dear Tire Doctor,

Is there a formula to determine what the maximum weight an 1100 x 20 load range H radial tire can handle (single) with a maximum speed of 5 mph. Sidewall states (Single 7390 @ 120 PSI cold) (Dual 6780@ 120 PSI cold)

Thanks, Gary

^ back to top

Dear Gary

Yes, there is such a formula developed by TRA.

Speed Range(mph) % load range Inflation pressure change
71 - 75 None No increase
66 - 70 None No increase
51 - 65 None No increase
41 - 50 + 7% No increase
31 - 40 + 9% No increase
21 - 30 +12% + 10 psi
11 - 20 +17% + 15 psi
6 - 10 +25% + 20 psi
2.6 - 5 +45% + 20 psi
Creep - 2.5 +55% + 20 psi
Creep 2)  +75% + 30 psi
Stationary +105% + 30 psi

1) Apply these increases to Dual Loads and
Inflation Pressures

2) Creep - Motion for not over 200 feet is
a 30-minute period.

Note: The inflation pressures shown in the referenced tables are minimum cold pressures for the various loads listed. Higher pressures should be used as follows:

A. When required by the above speed/load table

B. When higher pressures are desirable to obtain improved operating performance.

For speeds above 20 mph, the combined increases of A and B should not exceed 20 PSI above the inflation specified for the maximum load of the tire.

The maximum load and inflation capacity of the rim must not be exceeded.  We hope this helps you.

Best regards, Tire Doctor


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

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