Fitting the wrong tyres on the 911 GT3


The telephone rang. “Andrew, the car’s handling like a piece of sh*t.”

Spared the usual introductions, this could only my Porsche 911 GT3 driving chum who’d bought the car on my recommendation and, so far at least, had enjoyed every minute of it.

So we talked through the recent history. Had he hit anything, been up any kerbs on race tracks that could have deranged the suspension or failed to check his tyre pressures? No, no and no. All he’d done was swap his winter tyres back for the normal Pirellis the car usually wears, and taken the opportunity to replace the near worn-out rears. What could possibly have gone wrong there?

Quite a lot as it happens. I asked him to describe the symptoms to me. On dry roads the car appeared to lack traction and exhibit an hitherto undiscovered appetite for oversteer while in the wet the reverse happened: it just wanted to go straight on everywhere.

What had happened is that his dealer, an official Porsche dealer no less, had put the wrong Pirellis on the rims. They were the right size but instead of being of the hybrid road/track compound, construction and tread pattern of the Pirelli Corsas on which the car was developed, these were more conventional Pirelli Rossos, which duly created a total imbalance between the front and rear axles.

When challenged about this, the dealer said there were no Corsas available so it just slapped the Rossos on instead. You’d think that at the very least they might have thought to mention it. Instead they let a customer drive away a very finely tuned, thoroughbred sports car on completely the wrong tyres.

The more exotic and expensive your sports car, the more sensitive to tyre choice it will be and the more likely it will be that it’s been developed not simply with a specific tyre in mind, but with a specific tyre developed exclusively for it. Back in the land of ultra fast Porsches, the Michelin Cup tyres on the back of a GT3 RS and a GT2 RS are exactly the same size and wear exactly the same tread pattern. They are externally indistinguishable. But does that make them the same? It does not. In fact because of the colossal increase in torque brought by the turbocharged GT2 engine, its rear tyres are made from a different compound to improve their traction to make sure the torque goes where its wanted on the road and is not frittered away in pointless, expensive wheelspin. Porsche-sanctioned tyres carry specific ‘N’ ratings on their sidewalls showing they have been specifically developed for Porsche. If there’s no N-code on your Porsche tyre, it’s off the shelf rubber and can be expected to behave accordingly.

Nor is this a specifically Porsche-centric issue. The more extremely sporting your car, the more sensitive it is likely to be to the tyres it wears, a principle that extends all the way to Formula 1 where you’ll find entire chassis designed around the prevailing tyre specification of that season.

So if you own a properly engineered sporting car and it needs new tyres, find out first what it was designed to wear before heading down to Kwik-Fit and throwing on whatever’s cheapest from a well-known brand. If you don’t, you might save yourself a little money, but you may also be compromising the very talents that made you want to buy the car in the first place.

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