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TIRE PRESSURES: John Goodman says, “Most of us here (UK) have long junked our 15" wheels and run 16" or 17"
rims, but the less competitive drivers, or those who do a lot of city streets always keep the tyre pressures down quite a
bit. We run them up to 8psi lower than the handbook stated figures. My handbook says 35psi front and 32psi rear, so I
run at 30 front and 27 rear. On 245/50 or 225/55 x 16" this gives only very slight loss of handling at extremes of
cornering, but a very noticeable improvement in ride comfort. Has no effect on tyre wear either. This was a local
Jaguar dealer recommendation!!!” Note: perhaps a workable plan for those who value ride quality, but high continuous
speeds should be avoided without full pressure. Also note that lower pressure will provide less protection for your
rims, so don’t drive over curbs at speed.
Goodman’s car is an XJR-S, perhaps explaining those high tire pressure recommendations; the earlier plain-vanilla
XJ-S’s recommended rather low tire pressures (on narrower tires to boot!) and then recommended you go to higher
pressures if driving fast.
Regardless of the peculiarities of one XJ-S vs. another, one thing must be pointed out: the nearly religious devotion
many automobile owners pay to the “recommended tire pressures” is completely uncalled for. As long as you don’t
exceed the pressure limit written on the side of the tire itself, tire pressures can be varied considerably to provide the
owner with the desired performance and ride. If the handling balance doesn’t seem right to you, you can also vary the
front/rear pressure distribution to alter it.
For optimum tire wear, an excellent plan is to keep an eye on the wear at the edges of the tires vs. the center. If they’re
wearing faster in the center, lower the pressure; if they’re wearing faster at the edges, raise the pressure. This helps
configure the car for the owner’s driving style, since a driver that corners hard will scrub the corners of the tire more
and should be using higher pressure. It even helps compensate for drivers that dive into turns as opposed to drivers that
accelerate through turns, since the different styles will cause different tire wear patterns and correspondingly altered
pressures. Of course, bad alignment or other complicating factors can easily screw up the data.
Rob Ward tried to follow that guidance and ended up with tire wear in the center of the tread of his Goodyear Eagle
NCT 235/60ZR-15 rear tires despite low pressures. In response, someone posted this excerpt from a discussion on a
Porsche list: “My info, which is slightly counter-intuitive, is that fatter tires need slightly higher inflation pressures to
stiffen the tire carcass and prevent the centerline bulge that causes premature wear in the middle of the tire. Apparently,
the theory is that no matter what you do, fat tires will tend to higher diameters at speed in the middle of the tread, due to
centrifugal force when you twist it up. Increasing the tire pressure tends to stiffen the tire, preventing the center bulge,
rather than accentuating it as one might expect. It's actually enough more complicated than that that you need empirical
data to come to any kind of useful conclusion. I put 42-44 psig in my 245/45x16 rears and they have always seemed to
wear evenly.”
If you can’t tell, the XJ-S is nose heavy, and one would expect that the front tires will always need more pressure than
the rears -- unless you’re carrying sandbags around in the trunk.
Paul Bachman says, “The strangest thing happened yesterday. I was checking the tire pressure on the XJS and
discovered that they were quite low at about 28 PSI. After bringing them up to proper pressure (32 PSI), the minor (but
irritating) shudder in the brake system completely disappeared! Coincidence? I don't think so.”
WINTER TIRES: In some US states, you are required by law to have either snow tires or “all season” tires on your car
during the winter months. According to Bill Weismann, “For a tire to be labeled "All-Season" it must be labeled (By
federal law...) "M+S" on the sidewall.
“It's a lot more than tread design that qualifies a tire as an all-season; the main problem with High Performance tires is
that they have to be at somewhat high temperatures in order for them to "stick". An all-season tire usually offers two
features for it to work in the snow.
1) A tread compound that will remain pliable in the cold (below 40° F or so..)
2) A tread design that will work in snow.”
Europeans laugh at the idea of all-season tires. For people in places like Norway, an all-season tire means it sucks in all
seasons. If you get only a couple of weeks of inclement weather per year a set of all-season tires is a good option, but if