2000 XK8 Tyre advice

[Jag190902]

Hi everyone, I'm looking for some advice on rear tyres. I have 245/45 R18 on the front and 255/45 R18 on the rear with Vredstein Sessanta's on the front which are excellent and when I bought the car it had Vredstein's (not sure of the model) on the rear and the car handled beautifully. It failed the MOT on low tread so the garage fitted Kumho Ecsta's and since then the handling is awful at speed - the car tends to wander above 60mph and feels very vague. What do you recommend as I know these cars are very sensitive to tyre choice when it comes to handling and want something that is compatible with the Vredstein's on the front. I see that Vredstein does do an Ultrac Satin and a Quatrac Pro. Has anyone had experience of these? Would Pirelli P zero or Goodyear Assymetric 6 do the trick? Any help/advice would be greatly appreciated!

[mike020150]

One of the reasons that manufacturers recommend changing all four tyres at a time is to ensure that all four tyres are constructed to have the same slip angles. Crudely speaking, that means that, under load, the tyres twist through the same angles. Where front and rear tyres are from different manufacturers (or even different tyre types from the same manufacturer), there's a good chance that those angles will be different, even before you get to think about varying grip levels of different tyre compounds. It's not hard to imagine that different slip angles produce different driving responses, e.g. significantly greater slip angles on the rear could produce a less stable feeling car. Slip angle information is not commonly available from tyre manufacturers so the only way to eliminate the issue for certain is to stick with the same brand tyres on both axles. Fitting different brands will always be a lottery. Sometimes, you will win. Other times you will lose. Mike.

[JimMann]

I appreciate it can be a question of economics for many people these days, and we need to do everything reasonably possible to allow us to continue our passions and our hobbies at a realistic cost. However, it frightens me more than a little when enthusiasts stint on tyres, especially on something as powerful as a Modern XK. When one considers that the 4 small patches of rubber are all that lies between you, your car and the road surface (or not!), then it is comforting to know that those 4 patches are the best they can be.

Mike has very succinctly explained the technical issue with mixing different tyre brands and / or types across the 2 axles of the car and one of the reasons for sticking with a known quantity "premium brand" tyre on all 4 corners of a Modern XK is in order to allow the axle pairs to be replaced at different times - these premium brand tyres are rarely as expensive as some folks might believe - and therefore being in a position to replace them at least as a pair and knowing that you can get the other "matching pair" next year, or the year after, can make powerful economic and safety related sense.

I have cited it often before, but one of the most logical places to seek information on suitable tyres for your car is in fact the Kwik Fit company's website. Now I never advocate that one needs to purchase their chosen boots from Kwik Fit, but in the real world, one should always ask why some internet web sellers can be so cheap for the seemingly same tyre? The answer may well lie in the fact that they can access the tyres that are "seconds" as far as the vehicle manufacturers are concerned and you are not going to be able to follow up to any degree with them. There is lots of information on this Forum, and in historic articles in the JE magazine, about tyres, their manufacture, their quality assessment and some unknown pointers that enthusiasts really need to be aware of.

At the end of the day of course, it is about personal choice, but as you currently have a significant mismatch of rubber on your car and, by your own assessment, the handling is poor, it is perhaps now the time to make that correction taking on the advice you are seeking.

Apologies if this is not the advice that you wanted.

Jim M

[Jag190902]

Thanks both for the advice - very informative and thought provoking. I'm of the mind to replace the rears with the same brand as the front and I'll probably contact Vredstein customer services to see which they recommend as I was happy with this brand on the car when I purchased it and how they performed. Rest assured I won't be using mail order, but rather a reputable tyre dealer as I'm all too aware of sub-standard parts available on various internet sites and as quite rightly pointed out, there would be little comeback purchasing this way. I'll post a further update once I've made my final choice and how the tyres perform.