donqueen

My 2000 Boxster has 18" wheels but a 17" spare.

Is it worth carrying ?

Peter North

Even if the overall diameter of the spare tire in comparison is the same as that of the ones on the car, you'd have to be sure not to put the spare on the rear in case there is a drastic weight difference which might strain your differential.

Or you can carry a much lighter alternative in case you get a flat...

ep3_lol

Yes. The size is not terribly important. It's a temporary fix intended to get you to a spot where you can safely park the car until you get a tow or get it fixed. You shouldn't be driving on it long enough to cause any issues.