The car is a '94 Jaguar XJ6. Rear-wheel drive, with ABS. Tire sizes are identical, so I would think the diameter is too. I would think the ABS would compensate for differences in braking adhesion, if there is any, but maybe not. The Pirelli spare is relatively unworn; the Contis are about 3/4 gone, but they did just pass annual inspection, and I had hoped to get one more season out of them, because money is a little tight right now. But if it is a genuine safety issue, obviously I’m going to spend the money.