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Tires/Tire sizing

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Question
QUESTION: I have a 2007 Toyota RAV 4.  Three tire dealers said I could not buy the tires on sale because they were 235/70/16 and my specs called for 215/70/16.  They said the extra width would impact mileage severely.  So, I agreed to buy the 215/70/16 set (I really needed tires, my fronts were near bald).  When I got home, I realized they had installed 225/70/16 set.  Will this "harm" my car?  Will it impact my mileage or the life of the tire/wheel/alignment/speedometer/suspension?  The ride is great and the absolute smoothest I've ever had in the RAV 4.  Of course the dealer now says that there is absolutely no problem with increasing the width like this.  Any expert advice?

ANSWER: Susan,

While not using the original tire size sometimes causes issues, this is not one of those cases.  The 2 sizes are close enough not to matter.  For practical purposes, the size difference should cause no issues.  You will be traveling about 1 1/2 mph faster at an indicated 60 mph, so be aware of that .


---------- FOLLOW-UP ----------

QUESTION: Thanks,
You are very thorough and easy to understand in your answers.  I am comfortable keeping these smoother riding tires. But, I do have a follow-up.

What about the mileage indicator, would it be 1 1/2 miles behind every 61 1/2 miles or is it measuring by the 16' tire size? Is milege indicator calibrated to the rim/tire size of 16' or to the speedometer?

If speedometer, then for every 20 mph, I am actually traveling about 1/2 mph faster.  In one hour I would be traveling 1 1/2 miles more than the speedometer is registering.  So, is the 225 width affecting the accuracy of my mileage indicator?  By this equation, even at 40 mph I am gaining/traveling 1 mph faster and, if mileage indicator clocks with speedometer, not registering 1 mile every 40.  At highway speed of even 60 mph on a 600 mile trip this would amount to 15 miles shortage on the mileage indicator.

Answer
Susan,

Both your speedometer and your odometer will be affected by the difference in diameter.  They both are driven off the same cable out of the transmission.

And, yes, your math is correct.  As you can see, the difference is small, but it does have some  minor consequences.

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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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