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Tires/Tire size for cx9

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Question
I have cx9 Mazda I'm not driving fast and it's only like 12 km a day
I would like to change the tire from the size 245 50 r20 to 285 40 r20 due that the first one not available or if it ms available the price too high as well as my friend will give me this size for free because its available in his shop :)
My question did it will effect too much to my car ??
Thanks for your support

Answer
Ahmed,

First, the wheels are too narrow.  The allowable rim width for a 285/40R20 is 9 1/2 to 11, and your stock wheels are 8 1/2".  Using a rim that is too narrow results in the tires wearing in the center, and directional instability, especially in cross winds. In other words, the car will wander.

Second, even if you use the proper wheel size, it is hard to tell what affect this change will have as the difference between the 2 makes and models are going to have a HUGE effect.  But assuming EVERYTHING else is the same, directionally, this to going towards a harsher ride, less fuel economy, and better handling.

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Barry Smith

Expertise

I have over 30 years experience in the design, manufacturing, and testing of tires. I have served as the technical advisor to the "800" number. I have authored or co-authored many publications - usually without credit. I can answer almost any technical question, but please don`t ask me to compare brands. I probably have prejudices because of my work experience.

Experience

Member SAE (Society of Automotive Engineers) Member Tire Society (Tire Technical Organization) SCCA Regional Competiton License holder Authored many training manuals on tires, their care and use.

Organizations
SAE (Society of Automotive Engineers) SCCA (Sports Car Club of America) Tire Society

Publications
You may want to visit my web site: http://www.barrystiretech.com/index.html

Education/Credentials
I am a graduate of the University of Dayton, Dayton, OH - BME, 1971 I attended graduate school at the University of Akron - but did not complete my PhD - something I regret.

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