Understanding Your Tires

There are basically four types of tires seen on passenger cars and light trucks (pickup trucks).  These are the P-metric and ISO-metric tires most often seen on passenger cars,  and the LT-metric and “flotation” (not metric) tires seen on light trucks.  In some cases,  light trucks come factory-equipped with P-metric or ISO-metric tires,  but you do not see light truck tires as factory equipment on passenger cars.

P-metric is a US Tire and Rim Association (TRA) designation system with tire section width specified in metric millimeters,  but rim size specified in US customary inches.  Section height is given as a percentage to be applied to section width.  It will have a “P-“ in front of the numbers that specify sizes.  This size code is molded into the tire sidewall.  There is a specific set of load-inflation tables from the US TRA that go with these tires (and no others).

ISO metric uses exactly the same numbers to specify size,  but lacks the “P-“ in front of the numbers that are molded into the sidewall of the tire.  Section width is given in millimeters,  and rim size in inches.  Section height is given as a percentage to be applied to section width.  There is a different set of load-inflation tables to use with these tires,  from the European Tyre and Rim Technical Organization (ETRTO).

LT-metric refers to a US TRA class of light truck tires,  using the same metric section width,  section height percentage,  and rim size numbers,  as the P-metric and ISO metric tires.  Those numbers are preceded by “LT-“.  These LT-metric tires have their own set of load-inflation tables from the US TRA.

The “flotation” sizes are the final form of the old US customary inch-size designations.  These are inch measurements for overall diameter,  section width,  and rim diameter.  For the light truck application,  this size designation is followed by the letters “LT”,  as well as another letter designating the load range of the tire. These have their own load-inflation tables from the US TRA.

You must use the correct load-inflation table for the type of tires installed on your car.  These tables are downloadable off the internet.  See the section Load Inflation Tables below.

What Is On the Sidewall

Using the spare for one of my own passenger cars as the example,  here is how to read the pertinent information off the sidewalls of the tire.  The example is an ISO metric tire manufactured by Firestone at a factory in Brazil.  The basic tire size and application information is shown in Figure 1.  The same information is molded into both sidewalls (inner and outer).  Right under the brand name logo,  the code shown reads “175/65R14 82S M+S”.  There is no “P” and no “LT”,  so you know it is ISO-metric.

The tire section width is 175 mm from inner to outer sidewall (not the tread width,  but from bulge to bulge of the sidewalls).  The ratio of section height to section width is 65%,  meaning the section height is 0.65 * 175 mm = 113.75 ~ 114 mm.  Section height measures from where the bead seats down inside the rim edges,  out to the outer tire diameter.  The “R” indicates radial construction.  A “D” or a “B” indicates the now much-rarer bias-ply construction.  The rim diameter is 14 inches,  which is also measured down inside the rim edges,  where the tire bead seats. 

The “82S” tells you the service the tire is meant for.  The load index is 82;  higher index is heavier load capability,  but this varies from size to size,  so you have to use the load-inflation tables.  The speed rating is “S” (112 mph).  The “M+S” is “motor plus sport”,  which is advertising hype,  not really useful.

Figure 1 – Basic Tire Type and Size Information

Somewhere on the sidewalls will be a block of information that gives you the max load and max inflation values the tire is rated for.  Values are given in both metric and US customary units for both items,  as shown in Figure 2.  For this example,  the max load the tire can carry is given as 475 kg = 1047 lb,  and the max inflation is given as 300 KPa = 44 psi.  The max inflation would be “gauge” pressure above local atmospheric,  not absolute pressure.  Max inflation is driven by how the tire is constructed,  not necessarily by the max load it carries in the rating tests.   More below about that.

It is crucial to understand that these max load and max inflation pressure ratings do not necessarily go together (although they can),  nor are they the recommended inflations for any given car.  But,  combined with the data in the appropriate load-inflation table,  together they give you a very clear picture of what loads the tire can safely carry,  and what pressures are required to achieve that capability. If the tires are stock size for the car,  just use the inflation pressures listed on the car’s placard.  If not,  you must adjust inflations using either the max axle loads and the load-inflation table for the replacement tires,  or the load-inflation tables for both the stock and the replacement tires.

Somewhere else on the sidewalls will be molded the relative ratings for treadwear,  traction,  and temperature.  These are shown in Figures 3,  4,  and 5 for this example,  as treadwear “400” (Fig. 3),  traction “A” (Fig. 4),  and temperature “B” (Fig. 5).  These are less informative than often believed.

The treadwear rating is relative to what a cheap tire might provide under the same loading and service conditions.  Values seen are “100” (the cheap-tire reference),  “200”,  “300”,  and “400”.  The higher values are supposed to provide more miles of tread life before they are worn out.  However,  the “400” does NOT last 4 times longer than the “100”;  it only lasts a little longer,  most of the time!  The changes in tire composition (usually extra carbon in the rubber) that confer longer tread life also act to lower traction and perhaps temperature ratings. The higher the carbon loading,  the harder the rubber.

Traction ratings are “A”,  “B”,  and “C”,  with “A” representing the best.  I’ve never seen a “C” traction tire,  and I cannot tell much difference between an “A”-rated tire and a “B”-rated tire.   I don’t see very many “B”-rated tires,  anyway.  Harder tires at higher carbon loading break free and skid easier.

Temperature ratings are also “A”,  “B”,  and “C”.  “A” is the best,  but I have never actually seen anything but a “B” rating ( I do not use retreads).  Tires run hotter under heavy load,  especially in hot weather.  If they get too hot,  the risks of tread separation and blowout rise greatly!  Retreads really suffer from this,  as the vulcanization temperature for the bond joint of the new tread cap onto the old tire,  is necessarily lower than the vulcanization temperature for the old tire carcass.  Otherwise,  the tire would fail during the retread operation.  It is an ugly little fact of life that rubber heated above its vulcanization temperature re-liquifies.

The other thing you really need to know is how old the tire is.  That is because tire rubber degrades over long periods of time,  leading to the tire blowing out,  or otherwise coming apart catastrophically.  A lot of people call this “dry rot”,  but it is really the action over time of ultraviolet light,  plus ozone and other chemicals in the air,  degrading the rubber. 

Somewhere on the outer or inner sidewall,  but not necessarily both,  is a DOT code that will tell you when the tire was manufactured.  For the example,  I first looked at the outer sidewall and found “DOT E2EX 38B” plus a blank oval where the date code should be.  That is shown in Figure 6.  Then I looked at the inner sidewall and found “DOT E2EX 38B 3110”.  The date code is the “3110”. That is Figure 7.  

Figure 2 – Max Load and Inflation Ratings

Figure 3 – Treadwear Rating (“400” Shown)

Figure 4 --  Traction Rating (“A” Shown)

Figure 5 – Temperature Rating (“B” Shown)

Figure 6 – DOT Code Date Is Missing On Outer Sidewall (Empty Third Oval)

Figure 7 – DOT Code Date Is Present On Inner Sidewall Third Oval (Date Code 3110)

This 3110 date code means the tire was manufactured in the 31^st week of calendar year 2010.  This is currently late 2021,  so this tire was manufactured 11 years ago,  and has been in storage waiting to be sold,  ever since then.  As long as the storage was relatively cool,  and especially dark,  the long storage time should not matter that much.  But there is no avoiding the exposure to ozone in the air.

Unfortunately,  there are no hard-and-fast rules for how old a tire can be.  A sort-of general rule of thumb is that it should not exceed 6 to 10 years old,  in service on the car.  Storage doesn’t count much against that,  except for the ozone damage,  as long as it was stored inside and dark.  Myself,  if I haven’t worn this tire out in 5 to 6 years,  I will consider replacing it as “unreliable in service” due to age.

Tire Speed Ratings

The published speed range ratings look like the following table.  As a general rule,  do not exceed the speed rating for the tires you have,  even for a short transient.  If the weather is hot or the load is high,  stay well below the rated top speed.  What happens is that the tire rapidly gets too hot flexing as it rolls,  while simultaneously being stressed harshly by centrifugal forces.  This effect gets greatly exaggerated at high loads or in hotter weather,  and failure can happen in minutes,  or even seconds. 

TIRE SPEED RATINGS TABLE

Tire Speed Ratings  from https://www.readingbody.com/understanding-truck-tires-load-ratings-and-sizes/ as of 8-25-2021

·        L: Only apply these tires to off-road trucks that will not exceed 75 mph.

·        M: Use with temporary spare tires at no faster than 81 mph.

·        N: These tires have a maximum speed of 87 mph.

·        P: Tires with a P rating, cannot go faster than 93 mph.

·        Q: Q tires are usually winter studded or studless tires and should not exceed 99 mph.

·        R: For heavy-duty LT trucks, the rating may be R, which indicates a speed limit of 106 mph.

·        S: Family vans and sedans use the rating of S to indicate speed maximums of 112 mph.

·        T: Family sedans and vans may also use T-rated tires, which can drive up to 116 mph.

·        U: U tires can reach 124 mph.

·        H: Sports vehicles with H speed ratings can hit 130 mph.

·        V: Sports cars, sedans and coupes often use V-rated tires, which have speed ratings up to 149 mph.

·        W: Exotic sports cars use tires that can reach speeds up to 168 mph and have a W designation.

·        Y: The fastest speed rating — once Z which indicates faster than 149 mph — now is Y, which allows the tires to reach 186 mph.

For the example,  the speed rating is “S”,  corresponding to 112 mph on a passenger car.  That particular car (a 1998 Nissan Sentra) will not go that fast on level ground anyway,  so this isn’t much of an issue.  It won’t even go that fast downhill,  under most circumstances.  It’s a little 1.6-liter 4-cylinder!

Load-Inflation Tables

I got my copy of the four load-inflation tables as one document from the Toyo tire website:  https://www.toyotires.com/tires-101/tire-load-and-inflation-tables

On that website,  there is a link to the stored tables,  which can be downloaded as one single file in pdf format.  That link looks like:  Load & Inflation Table Application Guide

I recommend that you visit the indicated site,  follow the indicated link,  and download the pdf document that contains the four load-inflation tables all in one place.  That file also contains instructions for exactly how to use them.  These four tables cover P-metric,  ISO metric,  LT-metric,  and “flotation” tire types. 

There is an oddity to be aware of,  for dealing with LT-metric light truck tires.  The load index is often two numbers separated by a slash,  not just one.  The higher number pertains when there is a single tire and rim on the end of the axle.  The lower number pertains when there are two tires on two rims on the end of the axle.  The lower load index number represents a lower max load to be carried by each tire. 

The reasoning behind this is what the remaining tire can carry,  if one of the two on the end of the axle deflates.  The remaining inflated tire has to carry the load of the pair,  until the event is detected and the vehicle can be stopped for repair.  It can carry some overload,  but not a full factor 2 overload,  for a short time.  So,  the load index rating is reduced for that dual installation and deflation scenario.

There is another oddity to be aware of,  if P-metric passenger tires are fitted to a light truck.  Light truck-rated tires (the LT-metric,  and “floatation” with “LT” in the size) are typically built of tougher construction,  to withstand more routine abuse and more destructive driving conditions.  P-metric passenger car tires are just not built to withstand that kind of abuse.  While ISO-metric tires are generally a little tougher than P-metric tires of the same size,  they also still fall short of truck service. 

That is why the US Federal Motor Vehicle Safety Standards (FMVSS) demand that the allowable load ratings at any given inflation value be reduced by a factor of 1.10,  if a P-metric tire is to be fitted to a light truck.  If the factory tire on a light truck is a P-metric tire,  this factor 1.10 reduction has already been applied at the factory.  This load rating reduction of the ISO-metric tires is not so required,  but something like that would be highly recommended. 

Resizing Loads and Inflation for the 1998 Nissan Sentra

This is an odd case,  as the stock load margins are very low,  and this car has a history of hot-weather tire blowouts.  P-metric tires of this size (P-175/65R14 load index 81) are no longer so common;  instead the slightly-higher load index ISO metric tires (175/65R14,  load index 82) are now more common. 

The tire inflation placard for this vehicle (for the P175/65R14 size) lists 33 psi front,  29 psi rear for inflation.  The placard weight ratings are GAWR (gross axle weight rating) 1872 lb front (where the engine is),  and 1567 lb rear,  with a gross vehicle weight rating (GVWR) of 3413 lb,  which is always a little less than the sum of the front and rear axle ratings. 

Half the axle ratings are 936 lb front,  and 783.5 lb rear,  which the tires must bear during straight constant-speed driving on level ground.  At the placard inflations,  the tire load capabilities minus the axle rating loads (load margins) are rather small numbers: only about 58 lb front and 153.5 lb rear.  The plot is in Figure 8.  Note that the max load capability of 1019 lb is at the max inflation 35 psig.  

Increasing the rear inflation level to 30 psig raises that load margin to about 168.2 lb.  Increasing the front inflation to 34 psig raises the front load margin to about 74.7 lb.  These slightly higher load margins should help reduce the blowouts,  while still maintaining a handling difference of 4 psi,  same as stock.

Figure 8 – Tire Loads and Capabilities for P-Metric Tires on Nissan

The ISO-metric tires have a higher max inflation pressure (44 psig) than the pressure at max load (36 psig).  One plots this as constant load from the test data max load point,  up to the max inflation pressure point,  as shown in Figure 9.  If we choose 36 psig front and 32 psig rear,  we have larger load margins of 111 lb front and 164.5 lb rear,  at a front-to-rear handling differential of 4 psi,  same as stock.  

Figure 9 – Tire Loads and Capabilities for ISO Metric Tires on Nissan

Accordingly,  I am recommending revised tire inflation levels of 34 psi front and 30 psi rear,  vs the factory placard levels of 33 front and 29 rear,  whenever P-metric tires are used.  I also recommend the slightly-higher-still levels of 36 psi front and 32 psi rear whenever ISO-metric tires are installed.  There is no change to the 60 psi inflation for the “doughnut” spare. 

What this recommendation does is provide tire load capacities equaling or exceeding factory stock levels,  while preserving the front-to-rear inflation differentials.  That outcome is in accord with the guidelines for using the load-inflation tables.  The difference is that my procedure gets you the actual load margins for both the original stock and the revised tire selections.  You have more information.

Resizing Loads and Inflation for a 2005 Ford Focus

This vehicle came factory stock with P195/60R15 standard load (SL) tires of load index 87.  The stock tire inflation placard recommends 32 psig inflation all around,  no differential front-to-rear.  Stock tire load margins based on the load-inflation plot of Figure 10 are 158.5 lb front and 273.5 lb rear.  Hot weather blowouts were never a problem with this car.  There is no need to change the stock tire specification. 

Figure 10 – Tire Loads and Capabilities for P-Metric Tires on Ford Focus

Those factory stock tires are now hard to find,  but the ISO-metric equivalent size is available.  Those are 195/60R15 standard load (SL) tires of load index 91 and speed rating H (130 mph).   These tires are constructed stronger,  reflected in the larger load index,  and have a max inflation of 51 psig,  versus the stock P-metric tire max inflation of 35 psig.  

One would expect higher load capacities at pretty much any pressure,  and this is borne out in the load-inflation plot of Figure 11.  Those data lead to substantially-larger load margins of 236.5 lb front and 351.5 lb rear at the same 32 psig inflation.  These tires will be “tougher” against abuse at the same inflation,  than were the factory-stock P-metric tires. 

The recommendation is keep the factory stock 32 psig inflation (front and rear),  whether the tires are P-metric or ISO-metric.  The 60 psig “doughnut” spare is no different than it was.

Figure 11 – Tire Loads and Capabilities for ISO Metric Tires on Ford Focus

Resizing Loads and Inflation for a 2010 Toyota Prius

This vehicle came factory stock with P-metric P195/65R15 standard load (SL) tires of load index 89,  placarded for inflation at 35 psi front and 33 psi rear.  A plot of the load-inflation data in Figure 12 reveals the rather substantial load margins to be 144 lb front and 176.5 lb rear.  

Figure 12 – Tire Loads and Capabilities for P-Metric Tires on Toyota Prius

The replacement tires are now ISO-metric 195/65R15 standard load (SL) tires of load index 91 and speed rating H (130 mph).  The corresponding load-inflation data plot is given as Figure 13.  These tires increase the load margins to 177 lb front and 185.5 lb rear,  at the same inflation levels. 

Figure 13 – Tire Loads and Capabilities for ISO Metric Tires on Toyota Prius

Thus,  the recommended tire inflation levels need not be revised from the placarded 35 psig front and 33 psig rear.  The 60 psig “doughnut” spare is unchanged.

Resizing Loads and Inflation for a 1995 Ford F-150

This is another very odd case.  The original factory stock P-metric tires are now unavailable for this rather old pickup truck.  These were P235/75R15 extra load (XL) tires of load index 108.  The load-inflation plot for them is given in Figure 14.  Note the application of the 1.10 derate factor leading to the orange derated curve below the blue stock rating curve.  That derate factor is per the FMVSS for using P-metric tires on a light truck.   

Figure 14 – Tire Loads and Capabilities for Factory Stock P235/75R15 XL Tires

The front load margin calculates as 406.1 lb at 35 psig inflation,  and the rear load margin as only 84.5 lb at 41 psig inflation.  Max inflation for these tires is 50 psig.  That loading would be pretty much in accord with 6 occupants in the cab and the full rated 1000 lb of cargo in the bed of the truck. 

I could not get those tires recently.  LT-metric tires were available in this size,  but only with the block tread pattern that does not wear properly with the twin I-beam suspension this truck has.  The only P-metric tires I could find were slightly lower section height,  and (more importantly) only standard load (SL)!  Those were P-235/70R15 SL tires of load index 102.  The load-inflation table data for those tires are plotted in Figure 15 as the blue curve,  and then derated by a factor of 1.10 per the FMVSS as the orange curve.  Max load for these tires is at 35 psig,  and max inflation is 44 psig. 

Figure 15 – Tire Loads and Capabilities for P235/70R15 SL Replacement Tires

With these tires,  which are currently on the truck,  the front load margin at the placard 35 psig inflation is reduced from 406.1 lb down to 206.1 lb.  Technically,  that reduction violates the tire sizing guidelines given in the load-inflation tables document,  but it is still a fairly substantial positive margin. 

The rear load margin at the placard inflation of 41 psig falls from 84.5 lb to a -176.4 lb shortfall,  and THAT is a serious problem indeed!    A small positive load margin would obtain if the rear wheel load were reduced by about 200 lb at each wheel.  That corresponds to a 400 lb reduction in the rear axle load,  in turn corresponding to a 400 lb reduction in the truck bed load. 

The reduced section height also leads to a 3.2% reduction in outer diameter,  making the speedometer reading another 2 mph high at a real 60 mph on the radar,  for that reason alone. 

I have chosen to drive the truck this way at 35 psig front,  41 psig rear (whenever loaded),  but to reduce the truck bed payload from 1000 lb to 600 lb,  and to slow down to about 50 mph whenever 600 lb is exceeded.  This is primarily because they are SL,  not XL,  tires!  But I do not have to like this situation! 

I do not know if a better tire selection is really available,  but I did identify one represented in the load-inflation tables:  P245/70R15 XL tires of load index 108.  These would have a section width 10 mm (0.4 inch) wider,  but they fit the same stock rims.  The outer diameter is much closer to stock.  See Fig. 16.

Figure 16 – Tire Loads and Capabilities for P245/75R15 XL Candidate Replacement Tires

These tires,  if available,  would more-or-less restore the former load margins.  At 35 psig,  front load margin becomes 406.1 lb (same as stock).  At 41 psig,  the rear margin becomes 94.5 lb,  very slightly better than stock.  The only risk would be rubbing something in the front wheel wells during sharp turns,  because of the wider section width. 

The availability of LT-metric tires is far better,  in the stock size.  The problem is the block tread pattern wearing improperly,  something I have already experienced!  It really is a serious problem.  Assuming I could find non-block tread patterns in an LT-metric tire,  the only question is which load range (and the corresponding inflation levels) is appropriate.  Fig. 17 is for LT235/75R15 Load Range C tires.  No derate factor is required.  The load index is 104,  lower than stock.

Figure 17 – Tire Loads and Capabilities for LT235/75R15 Load Range C Tires

These tires require different inflations to meet the load margin needs.  At 45 psig,  the front load margin is near stock at 387.5 lb,  and at 50 psig,  the rear load margin is near stock at 85 lb.  This is acceptable,  but we are running the rear tires right at their max inflation limits.   That is not such a good idea!

Similar data for LT235/75R15 Load Range D tires are given in Figure 18.  The load index is 110,  higher than stock.  At front 45 psig,  we get the same 387.5 lb load margin as the load range C tires.  At 60 psig rear,  we get a greatly-improved load margin of 335 lb.  And,  we are still under the max inflation rating of 65 psig!  These would be quite acceptable,  if the non-block tread pattern could be obtained. 

Figure 18 – Tire Loads and Capabilities for LT235/75R15 Load Range D Tires

The LT235/75R15 Load Range E tires are plotted in Figure 19.  These have load index 116 (much better than stock),  and a max inflation of 80 psig.  

Figure 19 – Tire Loads and Capabilities for LT235/75R15 Load Range E Tires

At the same 45 psig front,  load margin is the same 387.5 lb as the load range C and D tires.  At the same 60 psi rear as load range D,  the load margin is the same 335 lb as the load range D tires.  These meet requirements,  but have excess capability (and expense) that are not needed.

The recommendation here depends upon the tread patterns available in this size for LT-metric tires.  If the non-block tread pattern is available,  then LT235/75R15 XL Load Range D tires of load index 110 are greatly preferred,  to be inflated 45 psig front,  and 60 psig rear,  when loaded.  Unloaded,  one can use 45 psig rear,  and it won’t break away so easily on a wet road.

Failing the availability of the non-block tread pattern,  we would prefer the original stock P235/75R15 XL tire of load index 104,  or as a backup possibility,  could accept the P245/70R15 XL tire of load index 105,  at 35 psig front,  and 41 psig rear when loaded.  35 psig rear unloaded handles better in the rain. 

Under NO circumstances,  should a standard load tire ever again be accepted for this truck!

Final Remarks

Those of you readers who are interested in refitting with larger diameter rims and lower section height tires,  you need to follow pretty much the same the same procedure as I did. 

Here is the procedure:

Go locate a copy of the load inflation tables for the four tire classes.  Look through them to determine where the data tables for each type really are.  Get familiar with them.

Find and plot the proper load-inflation data for your tires,  just like I did.  It will be a slanted straight line up to some max load point,  then a horizontal line from that max load point,  up to the max inflation pressure (if different).  Some tires have max load at max inflation,  most do not.

If you are looking at using passenger car tires on a light truck,  always divide the tire load values from the table by 1.10,  and plot them reduced that way (don’t mess with the inflation pressures).  Use those derated load data for your analysis.  You need not do that with LT-metric or “flotation” size tires rated “LT” for light truck application.  The FMVSS demand this derating for P-metric;  I’d highly recommend it for ISO metric,  too.

Look up or otherwise find your front and rear gross axle weight ratings.  Divide those numbers by 2 to represent the max loads applied to the wheels at the ends of the axles. 

For the stock configuration at the stock tire inflations,  compute for the front and rear the tire load capability minus the wheel load applied.  Those are your stock load capability margins,  front and rear. 

For your new configuration,  do the same thing for a number of candidate inflation pressures.  You are trying to equal or exceed the stock tire load margins front and rear,  at inflation pressures in the new tire that are (1) less than its max inflation,  and (2) that differs front-to-rear by no more than a psi or so,  from the stock difference front-to-rear.  Maintaining that front-to-rear difference is driven by handling effects:  it really is important,  especially in passenger cars.