Question: When I replace the tires on my vehicle, do
I have to use the same size as the originals?
Answer: On ABS-equipped vehicles, all vehicle manufacturers
recommend using the same size and aspect ratio tire as the original.
ABS systems monitor the rotational speed of the tires through
individual wheel speed sensors. Changing to an oversize tire
with a taller diameter than stock would cause the tires to rotate
at a slightly slower speed relative to vehicle speed than the
stock tires. Changing to a low profile tire with a shorter diameter
would cause the tires to rotate at a slightly faster speed relative
to vehicle speed. Though the difference either way isn't much,
it may be enough to upset the calibration of the ABS system and
have an adverse effect on its ability to detect and prevent skids.
Another reason for not changing tire sizes is because it can
affect the speedometer, odometer and transmission shift points
on a vehicle with an electronic automatic. Oversize tires will
make your speedometer read slower than normal (which may get you
a speeding ticket unless you have the speedometer recalibrated
to compensate for the change in tire size!). Smaller diameter
tires will make the speedometer read faster than normal, and increase
the mileage readings on your odometer at a faster than normal
rate.
All this doesn't mean you can't change tire and wheel sizes,
however. If you maintain the same overall tire diameter as before,
you can switch to larger wheels with a shorter aspect ratio tire.
This is the basic idea behind "Plus 1, Plus 2" tire
and wheel sizing.
Replacing a stock 14-inch wheel and 70 series tire with a
15-inch 60 series tire would be Plus 1. Plus 2 would be moving
up to a 16-inch wheel and possibly a 50 series tire. Plus 3 would
be going to the new 17-inch tire and rim combination -- which
could turn out to be a Plus 4 application if the vehicle originally
had 13-inch wheels.
ASPECT RATIO
The "aspect ratio" of a tire is the ratio of its
section height to its section width. The smaller the number,
the shorter the sidewall and the wider the tire. Low aspect ratio
tires started with 60 series some time ago, then progressed to
50 series and now 45, 40 and even 35 series tires.
Shorter aspect ratio tires (60 and less) are usually considered
to be performance tires because they lower vehicle ride height,
have a wider tread and put more rubber on the road to improve
handling. But the shorter the sidewall, the harsher the tire
rides.
A tire's ability to support a given load depends on its air
volume. If you go to a lower aspect ratio tire with a shorter
sidewall, the tire must be wider to maintain the same air volume.
If you just go to a shorter aspect ratio tire without increasing
width, the load carrying capacity goes down. That's why when
you go from a standard wheel to a Plus 1 wheel, the rim is usually
wider to accommodate a wider tire.
It's important to follow the tire manufacturer's recommendations
as to load capacities when going to larger wheel and tire sizes.
There's no hard rule that says you have to drop 10 points in
aspect ratio when increasing wheel size one inch, but that's the
general recommendation.
