I have a 2013 E350 Coupe with the typical 18" AMG double 5 spoke wheels. 8" wheel width front, 8 1/2 wheel width rear. Current tires are 235/40/18 front, 255/35/18 rear. I would like to soften the ride a bit, tone down the harshness. I am considering replacing current tires with 225/45/18 front and 245/40/18 rear - to provide a bit more sidewall flex. I can accept the fact that the speedometer reading will be off a couple of MPH. Any thoughts about this being feasible / desirable ?

I simply lower the pressure. As a rule of thumb I start with psi based on how much weight the tire can hold and how much is actually on it. If you only have 60% of the max load on it then I put 60% of the max psi. I usually put less psi in them but thats my starting point. How low I go depends on how it feels because they will get sloppy at some point. I think I currently have 34 up front and 30 in back. Lowering the back psi usually makes a bigger difference than front, but varies csr to csr. In my truck I run 12/9 psi front/rear, but it has larger tires so less psi needed. Size of the tire being another factor. Tread type and depth are also a big factor in ride. New tires always ride nicer thanks to more depth. Tread for racing ride harsher than those for cruising or "touring" tires. The tire size swich you mentioned may help, maybe, but I wouldnt bother. I suppose if you wanted softer sidewalls I get tires with a lower load rating. For example 93 instead of 97 load range.

I have a very similar query. Mine is a 2015 E400 cabrio with all 4 same wheels, 17 inches. Spec is 235/47R17. I am thinking of going a size bigger - 245/45 R17. Its obviously 10 mm wider and about 4 mm more in radius. I agree with Chevota that running lesser tire pressure will directionally compensate for the increased circumference. There is a 1.5% increase in the circumference on 245 vs 235. Manufacturing tolerance is about 0.5% and tread depth wear is also about 4-6mm. So I am thinking it will all be acceptable.
The car will count 100 kms done for every 101 kms done. I could live with that.
I am happy to be wrong - if anyone has a different experience, please do let us know here.

The lower pressure was to soften the ride. It doesnt compensate for height

I put 20s on my cab, 9 inch wide, 265/30 20 rear, 245/35 20 front. No rubbing or other issues, oddly speedo is now GPS accurate.

20 inch fronts

Ride isn't that horrible.

I’m a little late. I kept bending the AMG wheels on my E550. So I chose to keep same width but go up one aspect ratio size. 235/45 front, 255/40 rear. Bridgestone Potenza S-04 tires. Rears are fine but I had to put 3mm spacers on the fronts and find 3mm longer wheel bolts. Without them, the fronts rubbed the lower spring perch on the dampers. If you put spacers on, longer wheel bolts are required!

For a softer ride you need more sidewall. You can try a minus 1 configuration by changing to 17 inch wheels and 235/50 tires up front and 255/45 tires in back. Speedometer should stay correct. Just check for brake disc clearance before you buy new wheels and tires.

Nice bike collection, Busabuser! Steve

Be careful lowering tire pressure too much. It can detrimentally impact traction and accelerate tire wear. Steve