Tyre Speed Rating On Quattros Massively Conservative ?

[rt05492]

My 1998 A4 2.5 TDi Quattro Tip has a top speed of less than 130 mph and therefore according to a proprietary speed index chart - I could fit H rated tyres or even U at a push !

However, all web sites (and presumably Audi themselves) give a W rated tyre for this vehicle. That's 168mph and more than 40mph over what the car will do flat out, downhill and with a following wind !

Why the hell is it so high, is it something to do with the tyre construction needing to be suited to the quattro system ?

If not, then as an engineer this value seems illogical to me and I would tend towards the logic that says the top speed capability of the tyre should adhered to but as this is only about 0.00001% of the the working life of the tyre the chosen speed rating should reflect the usual working speed of the tyre which is within the H rating or anything between H and W ?

 

[aragorn]

I'm sure our car is on V rated tyres.

If the handbook gives its top speed as below 130 then i dont see a problem fitting H's. Your never going to legally do 130 anyway.

 

[rt05492]

Thanks. My top speed is covered by a 12 page 'Supplement to the Owner's Manuals entitled' entitled 'Technical modifications'. It appears to have been written entirely for the 110 kW 2.5 Tdi and covers fwd + 4wd, saloon + avant and manual + auto. Unfortunately all the auto values are 'not available at the time of printing'. However, the manual 4wd saloon gives 220 km/h or 137.5 mph compared to Autocar's 134 mph in 9.12.98, so at leat this is a reasonable claim (unlike many other manufacturer's). From that (manual has 6 speed and tip has 5), 125'ish seems perfectly OK for mine.

I do get a fairly regular run down a stretch of motorway where speeds approaching that can be held even around the curves - provided ones Killie pies, shocks and laser detector are working OK. Plus actual availability of the right size of tyre and the fact that, as you pointed out, they all seem to be down at V these days means I will probably go for Uniroyal Rainsport 2 in a V at the next change due to the current monsoon.

 

[aragorn]

I liked the old rainsport1, never driven on 2's in anger but i'm sure they'll be a good tyre.

V is more than adequate imo.

 

[jcb]

i think in sopme countries you are required to fit tyres whose rating "significantly exceeds the cars maximum speed".
I dont know what significantly means in this case, but given that the majority of vehicles have 155 limiters in place it seems a bit nuts.

 

[aragorn]

I've never been able to find out for certain how the law works in the UK, but as far as i can tell, you can happily fit lower rated tyres, so long as you dont exceed that speed, which is almost impossible to do legally in the UK anyway.

Insurance companies will sometimes insist on fitting tyres of a rating at least equal to those fitted by the manufacturer, although tbh thats just yet another bit to wriggle out of a claim.

I'm pretty sure Audi arent going to have specified W rated tyres for a car whos top speed is only 140. I looked in the manual but it all gets a bit vague, other than where is says if you fit winter tyres that are rated below the cars max speed, then dont exceed the tyres speed.

 

[jcb]

I believe you can invalidate your insurance if you run lower speed rated tyres with the exception of Mud and Snow tyres which tend to have a white highlighted speed limit rating on the side wall. on the basis that insurers will look for any excuse to not pay, it is probably not worth it, no cost saving as far as I can see

 

[Jonion]

The way tyres are speed rated is a bit of a misnomer, it's not all down to actual top speed of the vehicle but how it puts power down.
EG, you could have a car capable of only 100mph but with enough power/torque to pull your house down, that would need higher rated tyres than one that would do 110mph all day long but take a day to get there.

so you can fit the lowest rated tyre you think you can get away with but I'd rather over-rate them myself.