What a load of crock! You shouldn't be stretching tyres on rims. Tyres are designed, from a performance and SAFETY perspective, to suit a particular width rim, irrespective of the offset of the rim. The total wheel width & offset and tyre size package should be assembled with the optimum of performance and safety in mind. If a particular wheel / tyre combo won't fit under the guards, then something is wrong.And legally (roadworthy-wise) the rim / tyre MUST fit inside the guard, ie no part of the rim / tyre can protrude beyond the line of the bodywork..