Tires

Oh, if we could simply program life so things occur when convenient!

But, it isn't so...and thus I found myself at the tire store yesterday buying four new ones.  

I did a bit of research prior to buying the tires.  Thought I'd share with you some things I learned.   A couple of these things fall into the ridiculously obvious category once pointed out, but I had simply never given that much thought to the physics of tires.  (NOTE:  You don't actually have to read the math to get the points of the post.)

1.  The tires you remove and the ones you replace them with probably have different diameters.  For instance the tires I bought have a nominal tread depth of 23/64" (or, 11.5/32", to use standard tire tread measurement increments).  A tire is considered at the end of its legal life with a tread depth of 2/32".  It appears this means the tires I purchased, once at the legal end of their lives, may have reduced in diameter by 19/32" ([11.5/32  - 2/32] x 2).  This difference in tire diameter means the tires I bought with a 24.9" diameter at purchase will have a diameter of 24.3" at the end of their usable lives (calculations rounded, pun intended).

The circumference of the tire is a function of its diameter, the relationship being C = pi x d.  Estimating pi at 3.14 the circumference of the tire at purchase is 78.19 inches, whereas the circumference at the end of its useful life is 76.30 inches, a difference in circumference of 1.89 inches.

What is the practical impact of this?  Well, as they say, "your mileage may vary."  The odometer on your car is calibrated to rotations of the axle or drive train.  When that element completes a 360 degree journey, it is recorded in the car's odometer memory (I'm assuming the vehicle has an electronic odometer - but the principal is the same for a mechanical odometer.)  As such, this is an indirect measurement of the distance traveled because we are counting axle revolutions instead of actual distance traveled.  Some straightforward calculations show that at the beginning of the sample tire's life and the end of that life, using the circumferences as shown above, an actual mile on the new tire is 20 rotations of the tire less than an actual mile on the expired tire, yet the same number of rotations is being recorded as a mile.  Twenty rotations translates to 122 feet or .023 miles.  (There are other factors affecting this I haven't included here...including load in the vehicle.  Essentially this calculation is impossible to make, but these approximations show the point).  

Just by putting on the new tires (assuming the diameter is slightly larger than the tires you removed), the odometer will underestimate a "mile" on the new tires as compared to a "mile" on the old tires.  How will you notice this?  Simply due to the diameter difference you will see an immediate 2% drop in miles/gallon when you divide miles from your odometer by gallons used to drive them.

2.  But wait!  There is more!  Why do we put new tires on the car anyway?  Because the old tires are slick.  Being slick they don't hold the road as well.  In the lingo of the tire industry this is known as a reduction in rolling resistance.  The cure, of course, is to put new, deeply treaded tires on the car.  Voila!  An instant increase in rolling resistance.  Instant.  Know what that does to your gas mileage?  Yep...reduces it.  The benefit you get in road holding capability in your new tires is a detriment to your gas mileage.  In fact, the industry considers an immediate reduction in gas mileage by 10 to 20 percent to be absolutely normal when new tires are installed.

By now I'm sure you just can't wait to go buy new tires for your car.  Well, when you decide you can put it off no longer, do your research.  The resource I used most in deciding which tire to buy is tirerack.com. 

But let me share with you the benefit of my research:  all tires are NOT created equal and buying a tire because you recognize the brand might be the worst thing you can do.

In my case, I decided I wanted a tire that was quieter than what I was accustomed to.  I wasn't sure how much difference a tire could make...but I saw this used as a ratings measure and decided to make it key to my decision.  Result:  Today I found out how much I use accustomed road noise to gauge my speed when my eye isn't on the speedometer.  On the interstate I thought I was doing about 65 when in fact my speedometer read 80 mph!  I'm amazed by the difference!  The absence of tire noise with these new tires is extraordinary!

Here is a link to the details of the tire I chose.   Prior to beginning this search I had never heard of this tire company.  But you can tell from the survey results that a lot of people like this tire.  

One more thing...I bought the tires from Sears because the price was nearly the same and I could get them quicker.  I made the tire purchase online...and Sears sent me a text message almost immediately saying I could go to the Aiken Sears to have them installed (I couldn't see online that the tires were in stock in the store.)  When I got to the store the tires I wanted were on sale for less than I had purchased them on the internet.  Sears made all the refunds and adjustments and I got a better deal than I had settled for.  So - all of us internet savvy types need to be aware it pays to ask questions!

Speaking of asking questions, I want to ask you this:  Do you have your tires rotated?  If so, how frequently?  Tire rotation isn't a concept I grew up with and I wonder if it really makes a difference.  If you know, please let me know!  (By the way...I get free tire rotation at Sears for the life of the tires...).

Happy motoring!