Tyres! How much leeway do we have?

[sapphireblack]

I have some new Vredesteins Wintracs on E63 rims that could be used (on appropriate rims) on my X6 but the profiles are unsurprisingly different.

What i have are 245/40/19 on 8.5j rims and 275/35/19 on 9j rims. These will of course fit 19" X6 rims, the 9" square winter set up or the staggered 9 &10j summer wheels option.

Internet info suggests speedo errors of circa 5mph at 60mph. If theres snow on the roads speeds of that nature are unlikely so not a real issue. A ride height drop of circa 30mm may generate some transmission issues though, or maybe not?

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[X5Sport]

Cars are type approved with specific sizes - as shown in the tyre pressure plate. Anything different will take the car away from its type certificate. Technically that makes it illegal on the public roads.

Additionally you have a maximum of 3mm variation permitted between any wheel beyond which the X-drive will be under excess stress. If the tyres are ‘normal’ for a narrower rim then you would introduce a ‘lean angle’ whereby the sidewalk has to lean over and this reduces the rolling radius because the inner bead is a fixed diameter so the tread moves closer to the rim.

What weight rating have these other tyres got? Given they are off a smaller car they are unlikely to be suitable for a heavier X6. You can fit a higher rated tyre, but never a lower one.

Oh, and your insurance company will want to know and is very likely not to play as this would be an OEM unapproved mod.

Why take a risk? Put the proper rubber on.

[sapphireblack]

I had considered all your points, bar the weight aspect!

I think I'll stick to what is known to work and is approved. My growing collection of wheels is amusing but for the space they occupy

[sid10]

Hi no way would I be using these on a X6 not only will it be dangerous to use but no doubt invalidate your insurance if you have an accident ! I have the correct set of wheels with the correct tyres 255/50/19 Vredestein , 285/45/19 vredestein on style 258 like your own may px ? Tyres are dated 2014 .
sid10

[Spandex]

Cars are type approved with specific sizes - as shown in the tyre pressure plate. Anything different will take the car away from its type certificate. Technically that makes it illegal on the public roads.

This is simply not true, as I've mentioned before on this forum. The type approval ONLY applies to the manufacturer of the vehicle. Once you have bought your car, the only things you have to abide by are the Construction and Use regulations. And when it comes to tyres, the C&U simply state that the tyre must not be “unsuitable having regard to the use to which the motor vehicle or trailer is being put”. This is deliberately broad, to allow drivers to leagally fit whatever tyres are appropriate for how they will be using the vehicle. If you want to take the W rated tyres off your X5 to fit some S rated all-terrains because you occasionally take it offroad, then you are legally allowed to do that, as long as you never drive above 112mph.

Personally, I think changing the circumference of the tyres by the amount discussed by the OP is a bad idea, but I don't agree that it's even remotely illegal.

[X5Sport]

This.....

You need to get an approval certificate from the tyre manufacturer containing all the necessary technical information for any tyre or rim sizes that you want to buy which are not listed in your car manufacturer’s handbook. Anything different, such as size, must be checked first and given the okay by an expert. This, however is not necessary if the new tyres comply with the vehicle / EC type approval documents.

[Spandex]

Where are you getting this? Since when have tyre manufacturers provided ‘approval certificates’ when people want to fit their tyres? What experts are ‘giving the okay’?

Seriously, type approval is ONLY required by the manufacturer/their agent in order to sell their cars in the UK. It does not apply to owners of those vehicles. You are legally allowed to change anything on your car as long as your modification meets the C&U regulations.