Board logo

tyre speed ratings
blakep82 - 14/12/09 at 06:46 PM

been looking at tyres, my wheels (which came from another truck) had used tyres on them. 3 out of 4 are pretty worn to the markers.

tyres are 265 50R15, the only wheels i can find are cooper cobra. £70 each inc vat and next day delivery, i'm happy with the price, but not the speed rating

http://www.tyrespec.co.uk/products/?pattern=Cobra%20GT

appear to be rated S, which is 112mph? my car will probably never go above 100mph as i won't track day it.
how fast should i expect it to be capable of? how do IVA testers work out its speed and stuff?
those gear calculator things say i should get about 180mph, but wind resistance, friction etc doesn't come into those.

twybrow - 14/12/09 at 06:52 PM

Tyre speed ratings are used to determine if the tyres you have are up to the job you are asking of them. 112mph tyres are suited to cars with that kind of power/acceleration etc, not just those that top out at 111mph. Similarly, 200 mph tyres aren't just for cars doing 200mph, but for cars that are subjected to the sort of forces that getting to that speed might entail.

AFAIK, the IVA man is looking to see that your tyre speed rating exceeds that of your quoted top speed (again, they don't test your top speed, but it is an indication of how your car is likely to perform). This came from my SVA man, when I quizzed him on the topic....

smart51 - 14/12/09 at 06:53 PM

You have to have tyre ratings above the max speed of your vehicle, even if that is an absurd speed. The speed rating of a tyre is the highest speed that it will last for 1 minute from new. Yes it shocked me too. Don't think your 112 MPH tyres are actually good for 112 MPH. That said, if you stick to the 70 limit, I bet 112 MPH tyres are probably fine.

I also think its why the Germans are happy to have 155 MPH speed limits on their cars. V high speed tyres are extremely expensive.

twybrow - 14/12/09 at 06:58 PM

How about these?

blakep82 - 14/12/09 at 07:36 PM

so if i tell him a particular speed, how would he know if i'm right or not?

i kind of suspect these tyres might be wider than they should be for the wheels.
how do you measure the wheels accurately?

matt_claydon - 14/12/09 at 09:53 PM

quote:
Originally posted by smart51
The speed rating of a tyre is the highest speed that it will last for 1 minute from new. Yes it shocked me too.


Fortunately, whoever told you that was telling porkies! It's actually 20 minutes at max speed (v) having in one continuous test previously spent:

10 minutes at v-10 km/h
10 minutes at v-20 km/h
10 minutes at v-30 km/h
10 minutes ramping up from zero to v-30 km/h

The test is done with 80% of the max rated load on the tyre.

The pass criteria following the test are:

* A tyre which after undergoing the load/speed test does not exhibit any tread separation, ply separation, cord separation, chunking or broken cords shall be deemed to have passed the test.

* The outer diameter of the tyre, measured six hours after the load/speed performance test, must not differ by more than ± 3.5% from the outer diameter as measured before the test.

There are some slight different bits for very high speed rated tyres.

If anyone's interested, a Google for "ECE regulation 30" should get the full info.

jeffw - 14/12/09 at 10:22 PM

quote:
I also think its why the Germans are happy to have 155 MPH speed limits on their cars. V high speed tyres are extremely expensive.


I would think it unlikely you will find a newish German car capable of 155 MPH with anything less than Z Rated tyres on it. My A6 3.0Tdi has 255/35ZR19 98Y Vredersteins fitted which where £130 each. Rated at 186 MPH with a very high load rating.

[Edited on 14/12/09 by jeffw]

ReMan - 14/12/09 at 11:38 PM

Should'nt you be moving up to 17's or something and a lower profile for "racing"
?

blakep82 - 14/12/09 at 11:40 PM

i'm not racing

ReMan - 15/12/09 at 12:45 AM


OK for "racing style" then

blakep82 - 15/12/09 at 01:07 AM

i am considering a lower profile too anyway. really, i think i just want tyres that will get through IVA ok, and probably narrower than the current ones too.

235 40 r15 or something. can't find any of them lol
race trucks are 8.5 width i think. i think these wheels were the OLD ones they used before they went to 3 piece alloys

smart51 - 15/12/09 at 08:00 AM

quote:
Originally posted by blakep82
so if i tell him a particular speed, how would he know if i'm right or not?


I SVAed my Vortx on the tyres that came with the wheels. One of them had a 118 MPH speed rating so I put 115 MPH as the top speed. No questions were asked at SVA. It turned out at Silverstone that the car would do 120 MPH at the red line in top. Who'd have thought it?