I am a little confused about the metric tire system. i am looking at buying a set of 195/65/r15 tires for my daily driver car. I know the 15 is the rim size,but is the 195 the width and the 65 the height? say I get a a 205 instead of the 195,is this tire just 10 mm wider than the 195? thanks for clearing this up!!

 

Here is an explanation from Tire rack..

Tire Tech Information - A Guide to Tire Sizes

but basically the 195 is the tire width at the widest point and the 65 is the Aspect ratio which is the height from the rim.

So you are correct it is wider.

 

You are correct that the 16 is the rim size in inches. The 195 and the 205 are the width in millimeters. The 65 is the aspect ratio, or the height as a percentage of the width. So a 195/65R16 is 195mm wide and 126.75mm high (65% of 195). The R means radial.

If you want to go with a wider tire, but keep the same or close to the same height (for speedometer accuracy, etc), you go up on the width and down on the ratio. So a 205/60R16 will be 205mm wide and 123mm tall, or a 215/60R16 would be 215mm wide and 129mm tall, etc.

As an additional example, my truck comes from the factory with two tire options. An LT236/85R16 and an LT265/75R16. The LT means Light Truck. The 235/85 is 199.75mm tall, and the 265/75 is 198.75mm tall, so almost identical in height but 30mm wider.

 

How did the tire industry come up with this idiotic mixed inch and millimeter system? While I never paticularly liked the letter system ,G78-14 for instance, at least it didnt mix numeric dimensions, but included the aspect ratio, which sizes like 6.70-15 did not include.

Off topic, but how did we end up with metric spark plug threads in engines with otherwise all inch-based threads.

Jim

 

Keep in mind most of these dimensions are "nominal" and based on a specific rim width. They are close, but you will find some differences between manufacturers.

How we got mixed measurements....

Well, the Europeans used the same wheel sizes as the US for quite a while, but I'm not sure what the mix was between the English vs. Metric measurements for their tires.

Aspect ratio entered the fray with the "Wide Oval" tires in the mid '60s. Old tires like the 7.75 x 14s were referred to as "83" series for aspect. That seems correct as a 70 series is quite a bit wider for the same load tire.

The letter system was understood by almost no one.

Tires were early into the metric system, but retained the inch designations (despite Ford and Michelin and the failed "TRX" 390mm wheel size). Since about that same time performance tires were becoming common but we still had whitewall balloon tires for the pillow barges of the day, the combo of cross section-aspect ratio-rim diameter made sense. (to someone)

Sometime around then, light truck tires adopted the height/cross section/rim size in inches, such as 31-10.50x15. (which also makes sense)

Either way, you get a better idea of what the tire is all about than FR78x14 might give you, although I know exactly what that looks like, but I'd guess not many others do.

As for the old system with 6.70 x 15, everyone knew what that looked like to--at the time, which was before radials here, and before low profile, etc.

Even though Americans have 10 fingers, too many were unable to understand the metric system, despite the fact it is base 10. Thus, the move to metric stalled with some of the tooling and processes converted and the rest in the English system. Thus we have had over 3 decades of mixed vehicles.

But, you can get by in many cases because:

--Crescent wrenchs and adjustable box-ends work regardless.

--7/16 works on 11 mm
--9/16 works on 14 mm
--5/8 works on 16 mm (spark plugs--think this was an accident?)
--3/4 works on 19 mm (wheel lugs--another accident?)

Some others are close, but the above are right on.



Oh. We were talking about tires.....

 

Check this site out:

Tire Dimensions Made Simple - Discount Tire

 

Link goes to take off tires--a great site btw, but not Discount.

Try this for Discount Tire:

Reading the Tire's Sidewall - Discount Tire

 

Here is another site that explains tires and wheels; Car Bibles : The Wheel and Tyre Bible Page 2 of 2

 

Another interesting tidbit to know, even though bias plys are all but non existent now is that radials have the R in the size and use the metric system, bias ply's do not have the R and are usually listed in the inches such as 7.50-16.

 

thanks for the info,guys!! don't have time to check the links posted right now,but i will look at them in the future.

 

I wonder why it's not showing the link I posted.

I'll try it again.

http://www.discounttire.com/dtcs/infoTireMath.dos