Volkswagen GTI Forums - My Fast GTi banner
1 - 9 of 9 Posts

·
Registered
Joined
·
1,193 Posts
Discussion Starter · #1 · (Edited)
Okay, so in light of recent events here on MFG, I've decided to take it upon myself to write up a quick little thread on how to read a tire size.

We'll use the stock tire size on a MKIV 1.8T for reference. 225/45/17. Let's break this down.

225 - This means that the tire is 225mm wide from the outside of the tire wall all the way to the opposite tire wall. Please note: This does NOT apply to stretched tires (I can foresee those questions coming). This means when the tire is NOT mounted or the sidewalls are straight up and down when mounted, that will be how wide the tire is.

45 - This is the aspect ratio of the height to the width. It's basically saying that the height is 45% that of the width. 225 x 0.45 (or 45%) = 101.25mm. That's how TALL the tire is. The bottom of the sidewall (again, before being mounted) to the top of the tire (the tread).

17 - This is how big the tire is. What size wheel it will fit. Etc. This one is pretty easy.

There are other parts to reading a tire as well, but they shouldn't apply too much. Of course, there's the speed rating, but if you're trying to go fast you should already be well educated on tires and this write-up should have no bearing on you. Other small stuff to look for is a "P" before the series of numbers. This means it's a passenger tire vs. a truck tire. If you ever see something like 225/45/R17, that R simply means it's a radial tire which almost all of the common passenger car tires are. I don't remember seeing anything in a LONG while that has said differently.

If you'd like to know more, this is where I got a lot of this information (mostly because I couldn't figure out how to word anything without become too confusing) - http://www.ehow.com/how_2364482_read-tire-sizes.html

I hope this helps some people. [cheers]

Edit: If I left anything that's deemed important out, please let me know. I'll be happy to edit it in or correct any mistakes, etc.
 

·
Registered
Joined
·
11 Posts
That is exactly correct..Now if you want to know the overall height of the wheel and tire; using the example in the write up (225/45R17) is 20.99 inches in overall height. Take the aspect height in mm (101.25/25.4) = 3.99 + 17 wheel size equals 20.99. To convert mm to inches divide by 25.4.
I used to work at the Tire Rack back in the mid '90's.
 

·
Registered
Joined
·
23 Posts
This might be a stupid question, but tires always confuse me. I understand you always need the rim size to be the same, because that clearly isn't changing and the aspect ratio will change depending on how low of a profile you will want on the tire. As for the width (225 in your example) would that necessarily have to stay the same when you are looking to change your tires or are all you are really considered is the rim size. When I look at that link you posted they said the salesman is only really interested in the rim size. I understand most people in this forum are running larger wheels and a lower profile performance tire, but just for your average every day use does that width generally matter?
 

·
I'm Watching You
Joined
·
15,550 Posts
This might be a stupid question, but tires always confuse me. I understand you always need the rim size to be the same, because that clearly isn't changing and the aspect ratio will change depending on how low of a profile you will want on the tire. As for the width (225 in your example) would that necessarily have to stay the same when you are looking to change your tires or are all you are really considered is the rim size. When I look at that link you posted they said the salesman is only really interested in the rim size. I understand most people in this forum are running larger wheels and a lower profile performance tire, but just for your average every day use does that width generally matter?
for everyday use just stick with whatever width where the OEM tires

if you have wide wheels then you will need wider tire or small if you want stretched tires

all depends on the look you are going for

but it just for everyday use stay with the OEM tire size.

you can go wider but may have problems with rubbing

if you do low pro tire on the same size wheel you speedo will be off also
 

·
Premium Member
Joined
·
8,973 Posts
some light reading about tread wear, traction, and temp resistance for tires thats what the "240 or 400" or "AA" "A" ratings mean.

Treadwear Grades
UTQG Treadwear Grades are based on actual road use in which the test tire is run in a vehicle convoy along with standardized Course Monitoring Tires. The vehicle repeatedly runs a prescribed 400-mile test loop in West Texas for a total of 7,200 miles. The vehicle can have its alignment set, air pressure checked and tires rotated every 800 miles. The test tire's and the Monitoring Tire's wear are measured during and at the conclusion of the test. The tire manufacturers then assign a Treadwear Grade based on the observed wear rates. The Course Monitoring Tire is assigned a grade and the test tire receives a grade indicating its relative treadwear. A grade of 100 would indicate that the tire tread would last as long as the test tire, 200 would indicate the tread would last twice as long, 300 would indicate three times as long, etc.
The problem with UTQG Treadwear Grades is that they are open to some interpretation on the part of the tire manufacturer because they are assigned after the tire has only experienced a little treadwear as it runs the 7,200 miles. This means that the tire manufacturers need to extrapolate their raw wear data when they are assigning Treadwear Grades, and that their grades can to some extent reflect how conservative or optimistic their marketing department is. Typically, comparing the Treadwear Grades of tire lines within a single brand is somewhat helpful, while attempting to compare the grades between different brands is not as helpful.
Traction Grades
UTQG Traction Grades are based on the tire's straight line wet coefficient of traction as the tire skids across the specified test surfaces. The UTQG traction test does not evaluate dry braking, dry cornering, wet cornering, or high speed hydroplaning resistance.
The Traction Grade is determined by installing properly inflated test tires on the instrumented axle of a "skid trailer." The skid trailer is pulled behind a truck at a constant 40 mph over wet asphalt and wet concrete test surfaces. Its brakes are momentarily locked and the axle sensors measure the tire's coefficient of friction (braking g forces) as it slides. Since this test evaluates a sliding tire at a constant 40 mph, it places more emphasis on the tire's tread compound and less emphasis on its tread design.
In 1997, the UTQG Traction Grades were revised to provide a new category of AA for the highest performing tires in addition to the earlier A, B and C grades. Previously the A grade had been the highest available and was awarded to tires that offered wet coefficients of traction above 0.47 g on asphalt and 0.35 g on concrete. Today the grades and their traction coefficients are as follows:
Traction
Grades Asphalt
g forceConcrete
g forceAA Above 0.540.41, A Above 0.470.35, B Above 0.380.26, C Less Than 0.380.26

Unfortunately the immediate value of this change to tire buyers will be limited. Use of the AA grade will first be seen on new tires that are introduced after the standard was enacted and will then appear later on tires that have had the required wet traction all along, but were introduced when the single A was the highest available grade.
Temperature (Resistance) Grades
The UTQG Temperature Grade indicates the extent to which heat is generated/ or dissipated by a tire. If the tire is unable to dissipate the heat effectively or if the tire is unable to resist the destructive effects of heat buildup, its ability to run at high speeds is reduced. The grade is established by measuring a loaded tire's ability to operate at high speeds without failure by running an inflated test tire against a large diameter high-speed laboratory test wheel.
Temperature
Grades Speeds
in mphA Over 115, B Between 100 to 115, C Between 85 to 100

Every tire sold in the United States must be capable of earning a "C" rating which indicates the ability to withstand 85 mph speeds. While there are numerous detail differences, this laboratory test is similar in nature to those used to confirm a tire's speed ratings.
Unfortunately for all of the money spent to test, brand and label the tires sold in the United States, the Uniform Tire Quality Grade Standards have not fully met their original goal of clearly informing consumers about the capabilities of their tires. Maybe it's because tires are so complex and their uses can be so varied, that the grades don't always reflect their actual performance in real world use.
 
1 - 9 of 9 Posts
Top