The reason for the speed rating is mainly for the ability of the tire to
withstand heat and or operate with reduce heat due to its design.
Heat is the enemy of tires. There is heat from the belts flexing and
the just plain friction of the rubber molecules. Better rubber and belt
design reduces the heat allowing the tire to be driven at a higher rate
of speed continuously.
Regardless of how fast you drive, if you drive your Jaguar like the
typical Jaguar driver you will not be happy with the ride and handling
of an H rated tire. If you are and can’t tell the difference from the
proper V rated tire your suspension probably needs work besides new
tires unless you are so insensitive as to not be able to tell the
difference and there are people like that.
The Jaguar suspension was designed to use the specified tire. It will
handle and ride best with that tire providing your suspension is in good
shape.
I found it false economy as well as unsafe to drive H rated tires on my
Series III XJ6. Back then I was getting ripped off for $260 a tire for
the Pirelli P5 tires so I went down to an H or S rated Firestone the
tire shop told me would be ok as long as I didn’t drive 120 mph all
day. Wrong. I quickly found myself with out of balance tires. I’d
bring them in and rebalanced and a few weeks later be back. Couldn’t
keep them in balance. Now I was not driving 120 mph continuously or
even occasionally but I was hitting 75-90 mph on a 15 mile trip into the
city for short periods of time. The rest was city streets. The result
was noticed eventually. Heat build up was breaking the belts left and
right. When the guy got tired of replacing the tires because the belts
broke, he gave me my money back and suggested I use the tires rated for
the car.
The H tire is not rated for the side wall thrust nor the heat generated
by the suspension of the Jaguar at more or less normal driving 75 mph on
the freeway.
Forget the “speed rating” all that means is the tire wont build up
enough deadly heat to cause it to fail if driven that way on the
highway, but its not a rating fro straight-line driving either.
Cornering from freeway exits at any more than 25 mph is going to wear
that H tire out pretty fast. A lot of heat is generated in corners and
when the suspension is designed for a V rated tire, that means the
suspension is going to put V rated forces on that tire be it an H, S. V
or W rating.
I also found the H tire rode harsh and did not handle well. BTW if I
were an insurance adjuster and their was an accident and I found the XJ6
had H rated tires the first thing I would do is check the tires for
unusual wear, broken belts or bulges in the sidewalls as well as
carefully examining the skid marks for irregularity.
One other thing the H rated tire is not like to have is a soft enough
durometer rubber as a Pirelli P5 [the P5’s were like erasers they were
so soft]. If you look up tires specifications you fill find that there
is a rating for softness of the rubber [rated in durometer readings].
The Jaguar suspension and stopping distance is calculated on using tires
of about the same softness and tread design. Using a H rated tire with
a hard rubber will cause you Jaguar not to stop in the rated distance
would be my guess as well as providing less traction on wet roads.
But hey, do what you want its your money, hide and car. I just can’t
see where saving $20 a tire is worth the aggravation and risk of using
an H rated tire. If $80 is a breaker, you really out not to buy a
Jaguar, Even old ones cost as much as new ones to fix and any part on a
Jag cost twice as much as the same part on any other car. Tires are the
least expensive part of any Jaguar I can have Pirelli P4000 Super
Touring W rated tires put on my car for $105 balanced and including
disposal charge of the old tire. I don’t think I can save very much
using an H rated tire on the XJ40 and I know it was false economy on the
Series III.
Jay 90 VDP Majestic
wcmaness wrote:
In reply to a message from Phil Parrington sent Wed 11 Jan 2006:
Utilization of H rated, instead of Z rated tires would not be a
valid legal reason to deny an insurance claim. The ratings are
based on optimal performance at the maximum end - not normal safety
operation levels. Such safety issues fall under an entirely
different set of federal regulations for motor vehicle safety
requirements. Otherwise - all cars would be required to have Z
rated tires - because it’s safer in case you decide to speed -
right?
WCManess - 1991 XJ40/6 - Black/Grey - 4.0 liter
Arlington/Texas, United States
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