Tom, stick with the dimensions suggested for the ML55 - 18-inchers. You've got uprated brakes, and even if the wheels fit, there may not be enough heat dissapation with the smaller wheels.
Why choose cheese-y solutions for your supercar?? Go with the second set of wheels with the snows, but have them fit. They're available from the shmucks out there fitting on 22-inch wheels. ML55 wheels not only are larger, but wider, too.
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