"Tim+" wrote in message
news:1919761807.571994810.278464.tim.downie- But it's perfectly possible
to have a space saver with the same rolling
radius as a full sized wheel.

Of course it is, but then it wouldnt save as much space. Manufactures are
supplying space savers with a smaller radius.

As a matter of interest, how much smaller diameter are we talking about?

Oh - sorry. Forgot it was you. Get someone to explain rolling radius to

Maybe you ought to fact check space saver sizes rather than assuming that
they must be the same diameter.

You may well be right. I'm intrigued. I'll do some measurements. I always
assumed that the diameter was identical and that all the saving was in the
*width* of the tyre, on the maybe naive assumption that if you put a smaller
tyre on one axle, the car will want to go round in circles - irrespective of
whether the wheels are joined by a differential (front wheel) or are totally
independent of each other (rear wheel) - and will dip down at that corner,
putting less weight (and so less traction if the opposite wheel is a driving
wheel) on the diagonally opposite wheel.