If your car is say, a Volkswagen Golf
or Jetta the tire size is correctly represented as:
"225/45R17 91H" or possibly on some years "225/45R17 90H"
If the tire was for a VW Passat W8 the tire size would be:
225/45R17 XL 94H"
If this is a Volvo C70 the tire size would be:
"225/45R17 91Y"
"225/45R17 91H" or possibly on some years "225/45R17 90H"
If the tire was for a VW Passat W8 the tire size would be:
225/45R17 XL 94H"
If this is a Volvo C70 the tire size would be:
"225/45R17 91Y"
The first 3 sets of numbers are the
physical dimensions which are the same in all 3 of the above examples.
Physically these tires are interchangable. What is different is the last part
of the size, called the "Service Description" which tells us how much
weight the tire can carry at specific inflation pressures and how high a
maximum speed the tire is certified for.
If you put a tire with a service index
of 91H on a car that originally came with tires rated 91Y you would be lowering
the speed capability of the car and possibly other aspects of its performance
such as handling and braking as well. The "H" means the tire is rated
for 130 mph. A tire with a Y is rated for up to 186mph. Even at low speeds
however those tires might not behave the same way.
Similarly, if a car came with 225/45R17 XL 94H tires and you put 225/45R17 90H tires on it the tires, while physically identical in terms of dimensions will not be able to carry enough weight and can not be inflated to as high a pressure as the car requires. This could result in a catastrophic tire failure (blowout) and accident.
Similarly, if you change the physical size of the tire - say you switch to a 215/45R17 87Y you run into similar problems. This tire will have a smaller overall diameter meaning your speedometer and odometer will be inaccurate and your gearing will change, reducing fuel mileage. The tire is also narrower which combined with the smaller diameter makes for a smaller contact patch (the part of the tire in contact with the road).
Similarly, if a car came with 225/45R17 XL 94H tires and you put 225/45R17 90H tires on it the tires, while physically identical in terms of dimensions will not be able to carry enough weight and can not be inflated to as high a pressure as the car requires. This could result in a catastrophic tire failure (blowout) and accident.
Similarly, if you change the physical size of the tire - say you switch to a 215/45R17 87Y you run into similar problems. This tire will have a smaller overall diameter meaning your speedometer and odometer will be inaccurate and your gearing will change, reducing fuel mileage. The tire is also narrower which combined with the smaller diameter makes for a smaller contact patch (the part of the tire in contact with the road).
A smaller contact patch = less traction
in cornering and longer stopping distances. Also note the Service Description
has a load index of 87. This is lower than any of the 225/45R17 tires because
being a smaller and narrower tire it has a smaller air chamber.
The volume of
air a tire can carry determines how much weight it can support. Smaller tires
support less weight which means they need to be put on smaller cars. I wouldn't
recommend putting a tire with an 87 load index on a car that originally took
tires with a 90 or 91 load index, and you absolutely can't on a car that takes
a 94.
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