Tire Size Confusion???

J

joemark8

Registered
I read all of the POSTS on subject 245/50...235/60...or stock?
I need new tires soon and went to my tire guy and mentioned these different sizes that LOD members have been talking about. I presently have 225/60's on my 98 Base Model. The tire guy said that putting a different size on from the 225's I have now could cause my Odometer/speedometer mileage & speed to be off??? I thought thats a good point. He went on to recommend several tires, namely - Goodyear Eagles @ $100 each and Michelin Symetry @ $122 each. What is the real story LOD members? I have not yet purchased the new tires.
Joe;(
 
RE: Tire Size Confusion???

Let's do some math.

When we say 225/60-16 , we mean a tire than is 225 mm wide, has a 60 % width to height ratio and is mounted on a 16 " diameter rim.

Let's convert from metric to inches...

225 mm / 25.4 mm/inch = 8.85 inches

Our stock tire is approximately 8.85 inches wide.

Let's calculate the tire sidewall height(from rim to outside diameter)...

8.85 inches X 60% = 5.31 inches

Our stock sidewall is approximately 5.31 inches tall.

Now, let's calculate the tire diameter. The diameter is the sum of the rim diameter plus two times the sidewall height...

16 inches + (2 X 5.31 inches) = 26.62 inches

Our "stock" tire diameter is approximately 26.62 inches.

Understand that the speedometer/odometer is calibrated at the factory to stock size tires, you can follow that any major deviation of the tire size translates into more or less wheel revolutions which will determine the difference in the speedo/odo readings from stock. If you look at the 245/50-16 tire, you find the diameter is approximately 25.64 inches, a difference of approximately 0.98 inches in diameter. This is about 3.7% smaller. That's only about 2.5 MPH off at a real speed of 70 MPH.

Be aware that actual sizes do vary slightly by manufacturer. I've found Michelin's to be a little on the 'fat' side.
 
RE: Tire Size Confusion???

Can't say it better than JP did. Here's a great link that will graphically demonstrate the differences between tire sizes and give a good approximation of how much your speedometer will be off:

http://www.miata.net/garage/tirecalc.html

235/60s will make your speedometer read 1.8% too slow - when your speedo reads 60 mph, you're really travelling 61.1 mph.
 
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