All OE wheels have the wheel size in the casting molds or forging dies and you can see it on the inside of the wheel. For a 300E it would read something like:

6.5J x 15 ET49

The "ET" is the offset in millimeters. Since most Mercs carry the spare tire outside face down in the spare tire well, all you have to do is lift the cover, spin off the retainer, and read the numbers.

Duke