Let's start with the easy stuff... the SIZE... A common Lotus size is 175/70VR13... Okay, class what does the "175" mean? "THAT'S THE TIRE WIDTH IN MILLIMETERS...TEACHER..." Very good class... Now, what does the "70" mean? "THAT'S THE ASPECT RATIO, TEACHER... THAT MEANS THAT THE HEIGHT OF THE TIRE IS 70% OF ITS WIDTH...ABOUT 122 MILLIMETERS" Very, very good class. So, a 205/60VR13 is about 205mm wide and has about 123mm tall sidewalls. All of these measurements are taken with the tire mounted on wheel, more on how the wheel affects these measurements in a bit.

Now for a more difficult question... What does the "V" mean? "WELL, TEACHER THAT IS THE 'SPEED RATING'. A 'V' RATED TIRE IS FOR SPEEDS IN EXCESS OF 130mph." And the "R"? "THE 'R' IS FOR RADIAL CONSTRUCTION" Excellent students!

The last number, 13, is the wheel diameter measured at the bead of the tire (the inside 'lip' of the tire where it 'seals' to the rim). This measurement is in English, that is INCHES! Please don't ask why the first number is metric and this one English...

Recap. The first number is metric, millimeters for the overall width of the tire when fitted on a design width rim. The second number is the aspect ratio, represented as a percentage of the width. The next two letters are optional. The first is either a "Z", "V", "H", "S" or blank. These signify the speed rating. "Z" is for over 160mph, "V" for over 130mph, "H" for over 112mph, and "S" or blank is for 80mph. The next letter is only there if the tire is a radial, then it will have an "R". Without an "R" it is either bias ply or bias belted in casing construction. And again the last number is the wheel diameter in inches, usually 13, 14, 15, 16 or 17.

Now, let's backstep a bit, recall earlier I suggested that the wheel width affected the tire width, that in fact the tire width designation is based upon a design wheel width. This is in fact quite true. A tire's width will shrink on a narrow wheel (as in rim width narrower than the designed width). The narrow wheel's rims will pinch the sidewall inward, thus reducing the overall width of the tire. Conversely a wider wheel will "stretch" the sidewalls of a tire out, making the tire wider at the sidewalls.

The rule is that a tire's width changes approximately 0.2" (two tenths of an inch) for every 0.5" (half inch) of wheel width change. For example a 205/60-13 tire is approximately 8" wide on a design wheel width of 5.5". If one were to change wheels and mount the same tire on, say, a 7" wide wheel, then the overall tire width would be approximately 8.6". The 7" wheel being 3 x 0.5" wider than the 5.5" wheel. Again for each 0.5" add 0.2" to tire width and we have the original 8" plus 3 x 0.2", or 8.6". Of course the inverse applies. Going from a 5.5" wheel width to say a 5" width would reduce the overall tire width by 0.2".

It is therefore apparent that the design width of the wheel has a direct bearing upon what is marked upon the sidewall. So, the question arises, how is the design wheel width set.

The Tire and Rim Association (TRA) has established technical specifications that are based on engineering principles and approved practices. Standards for European tires are set by as similar body, European Tire and Rim Technical Organization (ETRTO). The design and measuring wheel width for a tires has been standardized by these bodies.

Thus for tires designated with an aspect ratio of 50 series or greater, the design and measuring wheel width is 70% of the tire's section width, rounded to the nearest 0.5". For tires with an aspect ratio less than 50 series, the design and measuring wheel width is 85% of the tire's section width, again rounded to the nearest 0.5".

When does 255=305? Consider the new Vette, it uses 255/50VR16 tires. The design section width of this tire is 10.04" as measured on a 7" wide wheel (10.04 x 70% per TRA standard). The overall diameter of this tire is 26.04". Thus the section height (tire height) is (Overall Diameter - Wheel Diamter)/2 = Section Height, or (26.04"-16")/2 = 5.02". So, the Section Height/Section Width = Aspect Ratio, or 5.02/10.04 = 0.50, a 50 series tire.

The TRA has approved the use of this tire on wheel widths ranging from a low of 6.5" to widest of 10". Let's consider mounting this tire upon a 9.5" wheel. That's 2.5" wider than the design wheel's 7". Recall that the tire section width will increase by 0.2" for every 0.5" of wheel width change. Therefore the 255/50-16's width would be increased by 5 x 0.2" or a full 1.0", or a total of 10.04" + 1.0", or 11.04". The new aspect ratio is therefore 5.02" (the section height) divided by 11.04" (the new width), or 0.4547, about a 45 series tire.

Now consider a tire designated as a 305/40VR16. The section width of this tire is 11.2", overall diameter is 26.2", a section height of 5.1". The design wheel width is 10". Let's mount this tire on a 9.5" width wheel, narrowing the section width by 0.2" to 11.0". The new aspect ratio would become 5.1"/11.0" or 0.4636, about a 46 series tire.

What does this all mean in terms of performance? Simply, put a wider wheel offers more stability for the sidewall of a tire. This increases steering response, handling and cornering stability. There is an outer limit to this rule, too much wheel width and the tire's tread will bow effectively reducing the contact patch. The other side affect is that ride harshness will increase. Here's a rule of thumb, the optimum wheel width is the same width as the tread width of the tire you're planning on using (no matter what the aspect ratio).

Now, we will discuss the DOT number that is embossed on the sidewall. Any street legal tire must have a DOT (Department Of Transportation) number. Having a DOT number indicates that the tire manufacturer has conformed with certain standards set by the Fed. The standards not only set minimum performance levels but means that the manufacturer has paid the Fed the proper taxes.

But, the DOT number itself is laden with information. Here's a typical DOT number that might be found on a European tire... DOT XT VE XKAw 09xx. What's all this mean?

The obvious is DOT, Department of Transportation, but next is XT. The two letters or numbers following "DOT" refer to the manufacturer and the factory location at which the tire was made, in this case Pirelli's factory in Sandbach, West Germany. The "VE" is a typical letter - number combination that refers to the specific mold used for forming the tire. The "XKAw" can mean anything... possibly even secret code from one Pirelli chap to another, but likely batch codes. And the final numbers refer to the date when the tire was made. In the case of our example "09" is the ninth week of "xx", where "xx" can either be a single digit if the tire was made prior to 2000 or 2 digits if after 2000. The "xx" is the year. A single "7" would be 1997, "03" would be 2003. This dating is important for those serious go fast boys who know that tires harden with age, thus tires that have a more recent date maybe more desirable.

So, let's break the code... figure 1 is list of the codes used for the factories used in the tire manufacturing.

Finally, more recently tire manufacturers have embossed some mystical numbers and letters on their tires supposedly rating their products for wear, wet traction and heat resistance. These number - letters are refered to as UTQG ratings. The numbers refer to wear (mileage), the higher the number the longer wearing, usually implying a harder rubber compound. The average is supposedly 100 and although never discussed a score of 100 is equivalent to approximately 20,000 miles. Thus a tire with a UTQG wear rating of 200 is about 40,000 miles.

The UTQG additionally rates for wet traction and heat resistance. These are rated in letters between "A" to "C", "A" being the better. An "A" wet traction rating is typically an indication that the tire has deep open tread pattern with lots of siping (the fine lines in the tread blocks). Often this type of tread pattern is unstable when driven hard on dry pavement. An "A" heat resistance rating can indicate a number of things: First, low rolling resistance due to stiffer tread belts, stiffer sidewalls, or harder compounds; Second, thinner sidewalls, more stable blocks in the tread pattern.

Armed with this sort of information tire shopping need not be so blind. Be assured go fast Lotus tires will typically have UTQG wear ratings of under 50 and have wet traction ratings of A or B and heat resistance ratings of B or C.

A

B

C

D

E

F

H

J

K

L

M

N

O

P

R

T

U

V

W

X

Y

0

1

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9

BFS = Bridgestone/Firestone Inc.
CGT = Continental General Tire Inc.
UGTC = Uniroyal Goodrich Tire Co.