The bolts for each type of wheel differ in length .
The hub section on most alloy wheels tends to be thicker than on a steel wheel , hence longer bolts are required to secure an alloy wheel than a steel one .
The problem would arise if you tried to fit a steel spare wheel using the longer bolts for an alloy wheel : the bolts would protrude too far into the hub . This will not normally cause a problem when the wheel is being fitted to the front of the car , but if long bolts are used to fit a steel wheel on the back of a Mercedes they will foul the shoes and other components of the parking brake contained within the small brake drum in the centre of the disc .
The solution , should you have the misfortune to suffer a rear wheel puncture on your Mercedes with alloy wheels and discover that you do not have a set of short bolts with which to fit your steel spare ? Take your steel spare wheel and fit it in place of one of your FRONT wheels using the long bolts ( this will be fine ; you can confirm this by rotating the wheel by hand whilst it is raised off the ground on the jack ) , you can then take the alloy wheel from the front of your car and fit it in place of your rear wheel with the punctured tyre .
Guido , I wholeheartedly agree with your point regarding punctuation : reading the OP's posts , which lack not only punctuation but correct spelling , made me feel quite breathless . Good punctuation is akin to good manners .
Although I have RAC roadside assistance by virtue of the fact that my car is insured with the RAC , I am fortunate that I have never needed to call them out . I remember being approached , as I was exiting a supermarket many years ago , by a gentleman trying to sell AA membership ; quick as a flash , I replied " I don't need it : I DRIVE A MERCEDES-BENZ ! " . The poor chap was gobsmacked by this and responded with a look of awe
, I gave him a toot and a cheery wave as I drove by a couple of minutes later in the Ponton