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
31st 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.
