FAQ: New Tire Sizes

[sweetmx3]

I have a 92 and I currently have for tires 185/65r14 and I was wondering if 205/60r14 would be ok for the car no rubbing or anything.

[JWMX3]

205-60 would be fine but i hope you are planning on getting different rims cus the smaller sidewall on the same rim will throw your speedometer... in that case.. you may as well upgrade to 15 or 16 inch.... just go to www.tirerack.com and find out the overall diameter of the tires you have now and make sure the new tires weather 15 or 16" have the same if not very very close overall diameter
make sense ? :p

[atlantamx3]

It shouldn't affect the height of the sidewall too much since the tire will be wider.

[mx-3xplosiv31]

The sidewall rating is just a percentage of height in respect to the tread width. so take 65% of 195mm and you get 126.75mm. now 60% of 205mm is 123mm. Now you need to figure your rim size in mm, and to do this, multiply the rim in inches by 25.4 (# of mm in one inch). so if you have a 14 inch rim, your rim in mm is 355.6mm. now, add the rim to the tire X 2 ( there is a sidewall on each side of the rim), and for the 65 sidewall we get 609.1mm. for the 60 series we get 601.6mm. now, you have to convert back to inches, to get the proper reading in mph, so you divide your total diameter by 25.4, so we get two numbers 23.98" for the 65 series tire and 23.69 for the 60 series tire. Now to get the circumference of the tire, you multiply the diameter by PI, or 22/7, and round the answer to the hundredth place. so for the 65 series we get 75.37", and for the 60 series we get 74.54". Now we know there are 63,360 inches in a mile, so you divide that by the circumference of your given tire application, and you come up with the amount of rotations in a mile your rim+tire make. so the 65 series rotates 840.65 times in a mile, and the 60 series rotates 850.01 times in a mile. Now multiply the mph by the amount of rotations in a mile, and you get the amount of rotations to travel, lets say, 60 mph. so stock is 50439 rotations in one hour, and the smaller tire is 51000.6 rotations in one hour.
Now to find out the difference in mph between the two tires, take the rotations in one hour for the stock size, and divide it by the circumference of the smaller tire, and you come up with only 59.33 mph, so you would be less than a mile an hour off at 60 mph on your speedometer. Not a huge difference if you ask me. Hope this helps. I also received this from another member, it is a little bit easier math solution to the equation.

Just take the ratio of the diameters (new/old) and multiply by the speed. In this case it would be 60*(601.6/609.1) = 59.26 mph. In other words if you change from old to new, when your speedo reads 60, you will be traveling at 59.26.


Aaron

p.s. maybe this should be posted in the suspension FAQ so people who have questions about plus sizing their rims can see it whenever they need to