Replacing staggerd wheels
I have 2014 Mercedes E550 with offsetting wheels
on front and back. What is the downside of
replacing them with aftermarket rims all same size
If the tire overall diameter is the same on both the front and rear you likely could do it, but if the tires are different diameters making them the same can cause significant problems with the traction control system causing numerous error messages and reduced performance. I would say DO NOT do it.
As an example, my 2004 Mercury Marauder has staggered tires. My wheels are the same diameter but the rear tires are of a different size from the factory. There is only about an inch difference in my tire sizes, but I can NOT go to same sized tires without causing issues with the traction control / ABS system. You can imagine that your E550 is vastly more technologically advanced than my 2004 Marauder. I am pretty sure it will cause you issues. Tires sized like this sure makes tire rotation interesting. Some tires can be rotated side to side, but if they are of a directional tread this is not an option.
The offset may also allow for different clearances between the wheel and the body. Not a good idea to alter Mercedes design here, changes may cause steering issues.
taver91 makes a good point that clearance issues could be involved also.
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