That's not the entire story. A tire's load capacity is directly
related to its cross-sectional area. Reduce the sidewhall height, and
width should be increased to compensate. (This is something most
ricers do not concern themselves with, I'm sure.)
You can also compensate for reduced cross-sectional area by increasing
air pressure, but that, obviously, has repercussions also.
That's not true, Dizzy. You can decrease the sidewall height and increase
the rim diameter, and keep the same tread width.
Within a few revolutions over an entire mile, a 225/55x15 and a 225/45x17
are identical. The tread is the same while the sidewall height is narrower,
but the circumference is so close that over a mile, the tire turns within 10
rotations of the same for each. So, narrower sidewall does not require a
wider tire. If one wanted to stay with the same rims, then you would be
right, sorta. If you went to a wider tire, you would have to reduce the
aspect ratio to keep the same overall diameter. The problem here is that
wider tires often times do not fit on the same rims -- a rim that's 7 inches
wide can only carry a tire that is relatively narrow in the first place, so
if one was inclined to replace a 16x7JJ rim with a 16x8.5JJ, then why not
get an 18x8.5JJ and get the tires to fit the rim properly and reduce the
sidewall height in combination with the wider tire so the overall diameter
remains within the specification for the car.
I just pulled a "standard" tire size out of my ass to illustrate the point,
if you had a stock tire of 205/60x16, you could replace it with a 245/40x18,
and have exactly the same -- for any practical purpose -- tire in terms of
diameter. The 16 goes 785.209 revolutions per mile, the 18 goes 784.248. For
any practical reason, these two tires are identical, except one is wider and
should fit the bigger rim than a wider tire on the original rim. The 16 tire
is 8 inches wide, the 18 inch tire is 9.5 inches wide. Both of these
examples would fit nicely on the rims I have described.
I haven't explored the fender clearances and undercarriage obstructions, but
I just wanted to illustrate that you can change the numbers around to get a
tire that fits the general spec of the original fitment, and do it very
If you wanted to stay with the same width of a rim, but change the diameter,
then you would retain the original tire width and simply decrease the aspect
ratio by 5 for every inch increase in rim diameter. If you had a 15x8JJ rim
that took a 225/55x15, you could replace the rim with a 17X8JJ and fit it
with a 225/45x17 tire, and nothing would be affected -- except in my case
the speedometer error was corrected from an error of about 6% to an error of
about 1% -- my car went from reading 80 and doing 75 to reading 80 and doing
Everything I stated is indisputable fact.
My point was about how load capacity is affected directly by the
cross-sectional area and the pressure, and how this should be kept in
mind when juggling wheel/tire sizes and widths.
I agree with Dizzy to a point. But I think that the sport package is doing
its job of reducing body roll and increasing road feel at the expense of a
soft and squishy ride.
I'd replace the run-flats with actual tires on the same rims and wait for
the lease to expire, then rethink plans to buy the sports package in the
My take on this is that the Sport suspension is designed to BE pretty much
what you are complaining about.
Luxo-boats are known for soft, supple rides. The sport package is intended
to firm the ride and increase the road feel. I'm not sure what the run-flat
tires do for the equation, but it could be worth your while to get away from
I had a 3 Series with the sport package that I also changed out the
225/55x15s for 225/45x17s, but I never thought the ride was "jarring." It
was stiff to be sure, but I like that quality in a car much more than the
wallowing-around-on-a-sack-of-pillows feeling. Granted, my 3 and your 5 are
a comparison of apples and oranges, but the overall point is that the sport
package is supposed to increase road feel and decrease body roll, which by
definition shoulld make a firm ride.
Just a thought to bring some perspective.
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