411
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