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It's built into the tyre.

You need a tyre with a "Z" speed rating. eg 235/45ZR-17

why does he need a Z rating? not doubting you, just curious as an R32 GTR is factory limited to 180kmh.

here are the speed ratings: http://www.tyresave.co.uk/tyrspeed.html

Krzysiu, I doubt you have 'R' speed rated tyres. Looks you got 18" wheels?

You probably saw something like 235/40/R18. In this case the R stands for Radial, the tyre's internal construction.

These days the speed rating is shown after this, along with the load rating.

For example 235/40R18 93W. Where the 93 is the load rating, W is the speed rating.

In the past the speed rating was shown within the tyre size. eg 235/45ZR17, like blind_elk said. Maybe some tyre manufacturers still do it this way.

http://www.tirerack.com/tires/tiretech/gen...neral/speed.jsp

Z - Rated = As fast as your car ca push em (technically suggests 240km/h+)

Y - Rated = Up to 300km/h

W - Rated = Up to 270km/h

V - Rated = Up to 240km/h

H - Rated = Up to 210km/h

U - Rated = Up to 200km/h

T - Rated = Up to 190km/h

S - Rated = Up to 180 km/h

R - Rated = Up to 170km/h

should have a letter on the tyre somewhere after the size...eg 235/45/R17 Y

or 235/45/ZR17 in Z rated case

Trust a Y speed rating over a Z

Y have been tested up to 300km/h and can travel at those speeds.

Z have only been tested to 240km/h, but are believed to travel over that speed (although they have NOT been tested over 240km/h)

Here is some info I got from a website a while ago...

Here is the rating information for speed:

M 81 mph 130 km/h

N 87 mph 140 km/h Temporary Spare Tires

P 93 mph 150 km/h

Q 99 mph 160 km/h Studless & Studdable Winter Tires

R 106 mph 170 km/h H.D. Light Truck Tires

S 112 mph 180 km/h Family Sedans & Vans

T 118 mph 190 km/h Family Sedans & Vans

U 124 mph 200 km/h

H 130 mph 210 km/h Sport Sedans & Coupes

V 149 mph 240 km/h Sport Sedans, Coupes & Sports Cars

When Z-speed rated tires were first introduced, they were thought to

reflect the highest tire speed rating that would ever be required, in

excess of 240 km/h or 149 mph. While Z-speed rated tires are capable

of speeds in excess of 149 mph, how far above 149 mph was not

identified. That ultimately caused the automotive industry to add W-

and Y-speed ratings to identify the tires that meet the needs of new

vehicles that have extremely high top-speed capabilities.

W 168 mph 270 km/h Exotic Sports Cars

Y 186 mph 300 km/h Exotic Sports Cars

While a Z-speed rating still often appears in the tire size

designation of these tires, such as 225/50ZR16 91W, the Z in the size

signifies a maximum speed capability in excess of 149 mph, 240 km/h;

the W in the service description indicates the tire's 168 mph, 270

km/h maximum speed.

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