I read your article “Unnecessary Caution” (page 1546) concerning tyres and speed ratings with interest as some months ago I fitted a complete set of new tyres at a cost of £182.92.
My car, a Marcos (3 litre model), originally had Avon 175 HR13 tyres. However, several years ago, all the tyre companies except one ceased production of the HR (up to 130 m.p.h.) rating on this size, and when the car came into my possession it was fitted with Avon 175 SR 13 (up to 113 m.p.h.).
Only Michelin with the XAS now make an HR tyre in this size but unfortunately the only comments of Marcos drivers with experience of this tyre were adverse.
I searched for some old Avon stock but none exists. I also requested advice from the tyre dealers and was given the option of 175 SR’s 185/70 HR’s or 195/70 HR’s. Of the low profile tyres the 185 made the car’s poor ground clearance of around 3 1/2 inches even worse and something I could not risk with driving 50 miles a day to work along the debris strewn M62. It was when discussing this problem with my local Tyre Services (Goodyear) dealer that I received the following shock.
I was told categorically that Tyre Services would not fit SR rated tyres to my car as a matter of policy, being a high performance sports car, and also that they had received a letter from their Head Office advising them that in an unspecified part of the country they had received reports of the Polite stopping sports cars and checking tyre speed ratings.
Accordingly I investigated further through the AA and was informed that fitr my car SR tyres should not be used. The AA’s Chief Engineer, I was told, was adamant that cars should not be under-tyred in any circumstances., and that although we have a 70 m.p.h. speed restriction the car has higher capabilities not only in speed but in cornering, acceleration and braking etc.
Since this argument seemed logical and I intend to drive On the continent I decided to fit Goodyear Rally Specials which at 195/70 HR i3 have the sante diameter as the 175 and so maintain ground clearance and performance capability but at a price.
In their letter the AA said that up to present (June 1977) they had no information that any Police cases had been brought purely due to the fitment of “other than specification” speed rating tyres. But the implication seems to be that “incorrect” speed rating may be added to lists of other offences when making a ease against a driver.
The only question which I feel needs to be satisfactorily answered by the tyre companies is Whether an SR tyre on a high performance car is likely to fail when subject to the accelerating, cornering and brake forces front such a vehicle when keeping to a maximum speed of 70 m.p.h?
Leeds DAVID RATTEE