functional difference: 18" vs 19 or 20"
 

Hi Gang,

I'm considering to buy an E70 (3.5d) and was wondering about the tyres. By default, they come with 18" runflats (styling 210 for those who care).

Is there any functional reason to upgrade to 19" or 20" runflats ? Apart from (perhaps) looking better and being an extra 1/2" higher off the ground ? Longer running ? Better wear ?

Adv[ Thanks ]ance.

O.

I'll answer the thread myself, as quite a few people have looked at it.

Functionally, the diameter of the tire makes no difference. (Not even to tire life, despite having a larger circumference, according to one mechanic I spoke to.)

What *does* make a difference though is the width of the tire. Fat tires have a larger surface area in contact with the ground that thin tires do, which equates to more grip of the road (ie better acceleration, braking, cornering, etc).

See tyre size: Information from Answers.com to decode what the "### / ## R ##" means.

X5 tires :-
18" = 255/55 R18 => 255 mm wide (front & back)
19" = 255/50 R19 => 255 mm wide (front & back)
20" - front : 275/40 R20 => 275 mm wide
- back : 315/35 R20 => 315 mm wide

Thus, 20" tires have 15% extra contact area over both 18" and 19".

Note: the one exception with the width/grip thing is driving in snow, where less contact area is better.

O.

If you plus one or plus two correctly, the overall diameter of the tire should stay the same. When you increase the wheel size, you decrease the tire wall size and that should keep things similar. If done crorectly that is.

Typically tires on smaller rims wear better and ride smoother. This is because most BMW's with plus sized wheels wear heavily on the inside of the rear tires due to suspension set ups and the taller sidewall of the original sized tire acts as a shock absorber over bumps. Widening the contact patch should only make the tire wear faster. Especially the rears.

Also, the contact patch simply changes shape when you add plus sized wider tires. It does not maike the patch larger as far as overall area is concerned. The size of the contact patch is vehicle weight divided by tire pressure so the wider tires will have a contact patch that is shorter front to rear and longer side to side, which should increase cornering grip.

Most 19 and 20 inch wheels weigh more than 18's as well, which will have some minimal effects on the way you car rides, handles, accelerates and stops. The larger or heavier the new wheel, the more drastic the effects will be.

It is matter of personal preference.
A bigger wheel and wider tire translates to stiffer ride, more tire ware and more problems when hitting a pothole. Also the bigger the tire, same as for the price.
If you drive sporty, and like hard cornering, go for 20's, if you want to play safe, go for 19's. Also be carefull since run flats are stiffer than regular ones.
IMO, If you live on a rainy place or snow, it will be better to have a spare set of 18's for winter driving, wide tires are not good for snow or for extremly rainy conditions. ( aquaplaning)
Let us know your final choice
good luck