The other day I was shopping for some tires A...

Dear Car Talk

Dear Car Talk | Dec 01, 1994

Dear Tom and Ray:

The other day I was shopping for some tires. A local wholesale outlet which sells three well known brands had very attractive prices. They also had one thing I found quite confusing. They had stacks of four tires of each brand side by side. All were marked 205 75R 14. The Michelins and Pirellis were the same height, but the General stack was at least four inches shorter. When I stood a General and a Michelin tread to tread, the General was about an inch narrower. I even checked the sizes twice and my vision (corrected, anyway) is 20/20. Now what gives here? Are these tires really the same size? Would running the smaller tires be dangerous to my health, happiness, and well being?

TOM: Good question, Chuck. The tires ARE the same size. At least, the sizes that matter are the same.

RAY: The "205" part of the tire number (205 75R 14) means that the tread--the part of the tire that touches the road--is 205 millimeters wide. "75" is the "aspect ratio," which means the "height" of the sidewall (the distance between the tread and wheel opening) is 75% of the tread width (which, in this case, would make the sidewall height about 154mm). "R" means the tire is rubber, as opposed to, say, wood.

TOM: Actually, "R" means it's a radial tire. And "14" is the wheel size, or the size of the opening in the middle of the tire.

RAY: And on the three tires you were looking at, all of these measures were exactly the same. But, as you astutely pointed out, Chuck, there were other differences.

TOM: One is carcass stiffness (that sounds disgusting, doesn't it? ....sounds more like a description of roadkill). Tire carcass stiffness is a measure of how stiff the rubber is when the tire is NOT inflated. So if the General tires had softer carcasses, the tires at the bottom of the stack would tend to get squooshed by the weight of the tires on top. So that's one reason the General stack may have been smaller.

RAY: The other factor is a measure called "molded beadwidth." The "bead" is the inner-most rim of the tire. It's the only part of the tire that actually touches the wheel (if you went to pick up the tire to carry it out of your garage, your hand would be holding one of the beads).

TOM: When the tire is inflated, the beads push out against the wheel--one bead pushes against the inside of the wheel, one bead pushes against the outside of the wheel. And they form seals against the wheel, and keep air from leaking out.

RAY: The "molded beadwidth" simply measures how much space there is between the two beads (one on each side of the tire) when the tire is manufactured....before it's inflated.

TOM: Obviously, you don't want too thin a beadwidth, or it may be hard to push the beads out to seal against the wheel. But you also don't want it to start out too wide, or you'll put unnecesary stress on the tire once it's inflated.

RAY: So General may have chosen a thinner molded beadwidth than Michelin and Pirelli. And that would explain why the General tire appeared thinner in a side by side match up.

TOM: So what does all this say about the tires made by General? Well, not much, really. Tire experts say the carcass stiffness has no effect on the performance of the tire once it's inflated. And the same is true of the molded beadwidth, as long as it's within reason, and not too thin or too wide.

RAY: So while there are other reasons why General tires don't "stack up" very well against Michelin and Pirelli tires, how they literally stack up is not one of them.

Get the Car Talk Newsletter