Should there be a 2 inch difference between a P-235-60-15 and a P235-70-15?
I friend of mine wanted to put 60's on the back and 70's on the front. When he mounted the tires the 70's were almost 2" taller.

On my 66, I have P-235-60SR-14 on the back, and P-235-70SR-14 on the front, these are Radial T/A's and they are the same height, 24". why are the 15's that belong to my friend different height?


Rocky

Hello Rocky,

Yes, the 235 60 15s are 26.1 inches tall by my calculations and the 235 70 15s are 28.0 inches tall.

Here is how I calculate tire dimensions. The width in millimeters is the 235 number. Divide by 25.4 to get width in inches. Multiply that by the aspect ratio; .6 or .7 in the two cases to get the height and multiply that by 2 and add the rim diameter to get the total height.

Actually, Rocky, your 235 60 14s are 25 inches tall and the 235 70 14s are 27 inches tall; you have two inches difference in the diameters of your tires, also.

Bruce

Bruce,

I understand what your saying, But I just went out and measured from the ground to the top of the tire and the front and back measure the same, right at 25" this is the strange part.



Rocky

You obviously had the car on the ground with weight on the tires when you measured them. The sidewalls will deflect with weight on the tires. This deflection will vary based on rim width, tire pressure and aspect ratio. Low aspect tires will deflect less. However, a radial tire still has to rotate the entire circumference, so that effective height is still the height you would measure without deflection. I'll admit that I am not a tire expert, but I think my theory is probably correct, and I hope I have my terminology correct. Try measuring the diameter of the tire from front to rear instead of bottom to top with weight on the tire.

I did the math, there is a difference of 1.8 inches by using the formula.
You are correct the measurement is different measuring from side to side, But I still only get a half inch difference in measurements!!! this is perplexing.


Rocky