Which full size spare?

[gmc15]

My X5 has 20" wheels with runflat tyres, so no spare wheel. I want a full size spare as I tow a van and if I get a puncture I would like to be able to put a spare on. The question is do I need the same size wheel as on the front of my car as the ones on the back are fatter and I assume that the fatter wheel would not fit on the front of the car?
Can you see any problem with continuing my journey with a thinner wheel on the rear?

Just after a bit of advice please

[sid10]

Hi they run in winter recommended by BMW 255/55/18 et46 winter wheels , 255/50/19, 9 x 19 , et48 winter wheels either would do although it will not be as proud as the 20" wheels . Just be careful that you dont end up with an E53 wheel as it wont fit different pcd of 72.6 against 74.1 on the E70 so wont fit on the hub .
 

[Graeme]

I'd be very wary of fitting a different size wheel to the other on the same axle - even though the rolling circumference is supposed to be the same, a 19" wheel and tyre is almost certainly different to a 20".  Furthermore the width and load rating of the wheels is different - and again I'd be wary of having a wide rear wheel on the front or a narrow front wheel, with a lower load rating on the rear.  Carrying two wheels plus a jack is impractical unless you feel necessary, e.g. if you are towing on unmetalled roads in the back of beyond where punctures are much more likely. Carrying two tyres is less weight but still takes up a lot of room and getting one fitted would be another problem The winter wheel configuration is for 4 identical wheels so the inequality issue doesn't arise.  Furthermore, wheels of different size, width and weight on the same axle may cause damage to the gear and transfer boxes.

I too tow a caravan.  I've done thousands of miles here and in Europe.  I carry and electric tyre pump and two cans of tyreseal to support the runflats fitted in case of a puncture.  I also have full towing and recovery insurance in the event of a totally smashed tyre.  Touch wood I've never needed either.

Hope these reservations help answer your question.

Best wishes,

Graeme

[gmc15]

Thanks, looks like 2 cans of tyre seal is the way I will go

[kjb1]

i see no issue atall with using a different diameter rim.  At end of day manufacturers use a space saver skinny wheel more often than not. 

Rolling circumference will be the same,  and will cause no issues.


Sent from my iPhone

[X5Sport]

When BMW provided spares on 19" equipped cars, the spare was a 17".  No problem at all, but as it was a different size the 80kph/50mph rule applies.  As said above, it's the rolling dia that matters.

My new car has a smaller rim spare straight from the factory.