Jaguar XJ-S. Service manual — part 104

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http://www.tirerack.com

and David Littlefield suggests

http://www.tires.com

“This is the website for Discount Tires Direct, an online version of Discount Tires stores. The Discount Tire site will
allow you to look up the right size tire for your vehicle..” Note that there’s an option for checking the local dealers or
Discount Tires’ mail-order service, and you want the latter; the pages for the local dealers only lists tires that are kept in
stock by the local dealers, and you want to consider special-order stuff for a Jag.

It is impractical to list the attibutes of particular tires in this book, because tire models change so often. All the tire
companies have been bought out by other tire companies anyway, it’s nearly impossible to tell which company is
actually making your tires any more.

TIRE SIZES: According to Mike Morrin, the pre-H.E. XJ-S was fitted with 205/70-15 tires mounted on 6” wide
wheels. A 225/60-15 tire would be effectively the same diameter and a little wider.

The early US-spec XJ-S H.E. was fitted with 215/70VR-15 on 6½” rims. Later models came with 235/60VR-15 tires,
still on 6½” rims. In theory these H.E. sizes are a hair larger diameter than the pre-H.E. tires, but there reportedly was
no corresponding change in suspension or speedometer.

If your car still has 70-series tires, when they wear out you probably should replace them with 60-series tires; the
additional width is beneficial for stability, and there is no appreciable disadvantage with modern tire designs.

Still later cars came with 16” wheels.

The tire size code works like this: The first three digits, such as 215, is the width (mm) of the tire at the widest point --
the middle of the sidewall. It is always rounded to a number ending in 5. The two digits after the slash, along with an
implied decimal point, gives the aspect ratio: the height of the tire above the rim divided by the width described above.
For the 215/70VR-15, this is .70, and the height of the tread above the rim is 215 x .70 = 150mm. If there is no aspect
ratio indicated within the tire size, the aspect ratio is .80. The R in the tire size merely indicates it is a radial tire; it is
sometimes omitted. The -15 indicates it fits a 15” wheel.

The one thing you normally don’t want to change is the overall diameter of the tire, since this would affect your
speedometer and odometer readings as well as your ride height. Changing the ride height can have adverse effects on
the suspension geometry, even if you don’t mind the altered ground clearance. To maintain overall diameter when you
go to a wider tire, you also have to change the aspect ratio to a lower value. It is easy enough to calculate the overall
diameter given the size; just multiply the width number by the aspect ratio, multiply that result by 2, and add the
diameter of the wheel (remember to keep your units straight!). However, remember that the number you started with,
the tire width in mm, was rounded to a number ending in 5, and may not even be that accurate. The aspect ratio is
likewise subject to some creative estimating by the tire manufacturer. Trying to draw conclusions about a quarter inch
this way or that is just silly. There are online tire size calculators that give you the results out to several decimal points,
but don’t be fooled into thinking that there is really any such accuracy involved. And remember that, once you figure
out the diameter, you’re gonna set the car down on this tire and squish it a bit, throwing all your calculations into the
wastebasket!

In general, tire manufacturers increase the width number by 20 while decreasing the aspect ratio number by 10 to
provide the equivalent diameter tire in a wider size; this is true even if the math doesn’t really come out right -- they
make the tires the right size and round the numbers to these sizes anyway.

Tire manufacturers list a “loaded radius” in their specs, the distance from the ground to the center of the wheel with
weight on it. This measurement is meant to include the deformation of the tire under load so the consumer can
determine the actual ride height, and is usually far more accurate than trying to use the tire size formula since there is no
rounding involved in the numbers given.

If you have access to the WWW, there is an online tire size calculator for determining which size tires will replace
stock sizes. The URL is

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http://www.dsm.org/Fun/TireSize.html

Today, you really don’t need to worry about arithmetic. If you want to move to a wider size tire, you can merely opt
for a “plus zero” size on any tire selection listing.

Regarding what will actually fit: In general, the widest tire you can fit on the XJ-S seems to be around 235 or 245,
depending on the profile details of the particular brand of tire. However, a slightly wider tire can be fit at the rear by
increasing the offset of the rear wheels. The XJR-S took this tact and used rear wheels with 33mm offset and is fitted
with 255’s at the rear -- but don’t try that without either going to the larger offset or flaring the fenders. Since you
don’t want to change the offset of the front wheels that way, this tactic requires two different pairs of wheels, and
rotating becomes problematic.

Within each wheel well at the rear, directly above the hub carrier is a rubber bumper attached to the subframe. At full
suspension compression, the top of the hub carrier smacks this bumper. The E-type guys (who have trouble fitting tires
wider than 180mm) remove this rubber bumper to allow the installation of wider tires. They then install rubber
doughnuts around the shafts of all four rear shock absorbers to provide a bump stop. This idea should work on the
XJ-S as well -- but you’ll need to increase the wheel offset to widen the tires in the inboard direction without widening
them in the outboard direction and rubbing the fenders.

TIRE SPEED RATINGS: If you live in some parts of Europe, you are required by law to fit tires with suitable speed
ratings for your car. In the case of the Jaguar XJ-S, that means a V rating or better.

Interestingly, the V rating has changed over the years. In the 80’s when many of our Jaguars were built, a V rating
meant the tire was rated for 220 Km/h, or 137 mph. Today a V rating means the tire is rated for 240 Km/h or 149 mph.
Just as well, since this rating better fits the capabilities of the Jaguar anyway.

The speed rating also used to be indicated within the tire size, as in 215/70VR-15; such were the specifications called
out in the glovebox and owner’s handbook for the author’s ’83. This has changed as well, though, and now the speed
rating is listed with the load index (see below) adjacent to the tire size, as in a 98V. There remains a special rating
called a Z rating that calls for the letter Z to appear within the size; sometimes this is called a ZR rating, since the Z is
followed by the R for radial. You can learn all about speed ratings and other tire specifications from dozens of places
on the internet.

For Jaguar XJ-S intents and purposes, a tire with a speed rating of V, W, Y listed with the load index and/or a Z in the
size is acceptable. Any lower speed ratings (H, U, T, S, whatever) will be illegal in some areas and should be used with
caution anywhere else.

Speed ratings are intended to indicate suitability for Autobahn-style driving -- continuous high speed. Generally, the
limiting factor is heat buildup in the tire. High-speed-rated tires either run cooler or are made of higher-temperature
material, or both. Much of the heat generated is dissipated to the air inside the tire and away through the rim, so inner
tubes invalidate the ratings (the flexing of the inner tubes themselves adds to the heat buildup). There are also concerns
about how patching holes affects speed ratings; it is recommended that the owner check the literature that comes with
the tires before opting for a low-cost “plug” repair to his tires.

Since the flexing of the tire is what generates the heat, tire manufacturers usually achieve a high speed rating by making
the sidewalls stiff. Therefore, the high-speed-rated tires drive differently than normal tires, even at low speed. For this
reason, many recommend the V-rated tires even to those who don’t drive fast. This is especially true for the XJ6 and
XJ-S, because the soft suspensions get downright mushy with the softer, lower rated tires. There are reports that S-
rated tires used on these heavy cars will flex so much that they will wear out quickly.

The heat buildup due to tire flexing is also the reason you must have fully inflated tires when driving fast. Blowouts
aren’t caused by the tire pressure being too high; they’re caused by the structure of the tire failing due to excessive heat.
If you’re going to go fast, it would be better to err on the side of too much tire pressure than too little.

As the tread wears, the speed rating of the tire will actually improve. A thinner carcass generates less heat when flexing
and dissipates heat better. There have been promotions claiming street tires were successfully used for racing; in these
cases, typically the outer half of the tread was shaved off before the tire was even put on the car.

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The Pirellis and Goodyears that came with these cars were designed with a luxurious ride in mind. Most 60-series tires
with a V rating, however, are designed with performance in mind, which makes sense to everyone except a Jaguar
owner. The original Pirellis and Goodyears are NLA, so any suitable replacement tires are likely to result in better
traction and handling but perhaps at some sacrifice in ride quality.

TIRE LOAD INDEX: The load index is a number between 0 and 270 that indicates the load-carrying capacity of the
tire at max pressure. There doesn’t seem to be any rhyme or reason to the load index numbers themselves; you have to
find a chart to determine the rating in pounds or kg.

Iain Burgess says, “The 89 model owners manual says we need a load rating of 98 (745kg) for a 235/60R-15 and 97
(730kg) for the 215/70-R15.” It’s unknown why the requirement would be different for different size tires on the same
car, but the skeptical among us would suggest it’s because they are promoting a particular brand of tire rather than
providing an honest minimum specification. The fact that their specs would seem to be far in excess of what would
reasonably be expected may lend support to this theory: a load index of 98 means the tire is rated for 1653 pounds -- far
more than adequate for each corner of a 4000-lb car with 800 pounds of people and luggage in it.

Remember, though, that most of us don’t run our tires at the maximum rated pressure; we run them at pressures
considerably lower than that, so some margin in the load index would be a good idea. If you install tires with a load
index in the 80’s the rating is marginal for this car and you might want to keep your pressures up -- especially when
loading lots of people and luggage.

This is yet another reason to upgrade to 16” wheels. Burgess claims it’s far easier to get tires with adequate load
indexes in 16” sizes.

TIRE WEAR RATINGS: They sound useful, don’t they? Gary Penovich says, “You can't compare the wear rating
from one manufacturer to another. For some stupid reason, that number is relative only to tires from the same
manufacturer.”

UNIDIRECTIONAL: Some modern tires are “unidirectional”, meaning they are intended to roll in only one direction.
They have a distinctive tread pattern intended to efficiently clear water at speed. This must be a good idea, because
racing rain tires all look like this. The disadvantage is that you cannot rotate tires left-to-right without pulling the tires
off the wheels and remounting. Since rotating front-to-rear is usually what you want to do anyway, this doesn’t present
a problem.

Obviously, if you use a unidirectional tire as a spare it’ll be on the correct side on one side of the car and wrong on the
other side, so be careful about which side of the car you run over nails with! Seriously, they’ll work fine rolling
backwards, they just won’t clear water as well at speed. If you have one rolling backwards, be careful in the rain.

Typically, all four tires in a set of unidirectional tires are actually the same tire. They must be installed so they all roll
forwards, so the right side of the tires on the right side of the car will be visible while the left side of the tires on the left
side will show. This means the labelling on the sidewalls is the same on both sides of each tire so it looks the same on
both sides of the car; the manufacturer cannot put raised white letters on one side or some such and give you the choice
of mounting them facing inward or outward.

Despite the similar appearance on each side, the serial number -- which includes the date of manufacture -- is only on
one side of each tire, so they’ll be easily readable on one side of the car but you’ll need to look on the inside on the
other side of the car.

ASYMMETRICAL: Asymmetrical is not the same thing as unidirectional. Asymmetrical tires are different on the
inner edge than on the outer edge. Sometimes it’s just the tread pattern -- closer to a “slick” on the outer edge for
cornering with bigger grooves at the inner edge for water clearing -- while some tires actually have different rubber
compounds on the different sides.

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If your tires are just asymmetrical, you could rotate them all around the car since once mounted on the wheels correctly
the inner edge will be the inner edge whereever you put them. However, most asymmetrical tires are also
unidirectional. This means you cannot rotate them side-to-side, period; while you could rotate the unidirectional tires
side-to-side by remounting them on the wheels, doing this with an asymmetrical tire will put the inside edges on the
outside. Asymmetrical-and-unidirectional tires must actually be different tires on the left and right side; if ordering
online or some such, be sure to order two lefts and two rights.

TIRE AGE: You might want to be careful about buying tires that have been sitting on the dealer’s rack for several
years. David Littlefield says, “If you read the fine print in the tire warranty, it will tell you that the warranty for tread
life and defects is only good for a maximum of five years from the date of manufacture. Thus, when you buy a three-
year-old tire your actual warranty is only two years.”

The concern about buying old tires may be news to some people, but it’s a legitimate concern here. The tires that fit the
XJ-S with 15” wheels are fairly unique; in fact, the 235/60R-15’s appear to be a Jaguar-only application. Hence, it’s
entirely plausible that the tire dealer -- or the supplier he gets his tires from -- has had that set of tires sitting around for
too long waiting for a Jaguar owner to sell them to. While buying tires for your Toyota might not raise such concerns,
you might want to pay attention with the Jag. Fortunately, the tires that fit the XJ-S with 16” wheels appear to be
shared with Camaros and Firebirds, so old stock can be expected to be less of a problem. Opting for 225/60R-15’s may
avoid problems as well, since this size is reportedly used on Mustangs.

The date of manufacture is also included in the data written on the side of a tire. Prior to 2000, the last three digits
following the DOT serial number were two digits for week of year and one digit for year. If there is a little triangle
after these three digits, the tire was made in the 1990’s; if not, it was made in the 1980’s. Starting in 2000, it became
the last four digits, two digits for year.

15” TIRE AVAILABILITY: All of the above seems like a lot to think about, but when you get to the tire dealer and
actually try to buy tires you may find things get a lot simpler. The tire companies make tires to fit cars rather than in a
general range of sizes and specifications, so the tire dealer just types in your make and model and up pops the tires
intended for use on your car. For example, for the ’83 the computer will list all the 215/70R-15’s as original size,
235/60R-15’s as a “plus zero” size, and various 16” tires as “plus one” and 17” tires as “plus two”.

Once on that computer, you’ll find that most of the 235/60R-15’s have a load index of 98. That’s correct, the tire
companies intend them to fit Jaguars. Interestingly, many of these tires are not V-rated; they presume you don’t need
the speed rating in the US.

If you have trouble finding 60-series 15” tires, Wade Ramsey suggests: “215/65-15's will work as well. What I mean
by "work" is the outer diameter for both tires is almost identical. The 235/60-15 OD is 26.10 inches, 215/65-15 OD is
26.00 inches, while the OEM 215/70-15's OD is 26.85 inches. The reason I mentioned this is simply to let others know
that there is an abundant supply of 215/65-15's available in H and V speed ratings.”

Another option is to go to the plus-zero size tires appropriate for the pre-H.E. -- 225/60R-15. These tires invariably
have a load index of 95, apparently because they are intended for a Mustang and that’s what Ford says is the load index
needed on that car. This is still a load rating over 1500 pounds per tire, seemingly plenty for an XJ-S and possibly even
for a Jaguar saloon. And many of the tires available are V-rated or better, often Z-rated.

As of this writing, this author would recommend you go to the 225/60R-15 when you get a chance. Not only is the
selection far greater, but there is less chance that the tire model will be discontinued within six months of buying the set
leaving you screwed when you tear up one tire on a pothole.

It’s easier and often cheaper to buy tires with adequate speed and load ratings for 16” wheels than for 15” wheels.
Since this is likely to be more and more true in the future, anyone considering buying new wheels would be well-
advised to opt for 16” wheels.