So I've done a lot of searching on this site and have found a lot of good information. I'm fairly well versed in Audis (I have an A5) and their wheel fitments but i am new to the Mercedes. A friend of mine has a '13 c250 coupe and she is about to pull the trigger on 20's and H&R sport springs.
The specs are 20x8.5 et 40 and 20x10 et 40 on 245/30's and 275/25's. I thought I would see what y'all's thought were on this set up.
Thanks in advance for the input.

In my opinion 17/18 inch rims on this car are having perfect look, 19 is OK. 20 inch too much ...
Remember you will need to rotate much larger/heavier rims/tires with this initially weak 4 cylinder engine. It will get back to you with much worse mpg and performance overall.

Personally, I think 19s are the best and 20s are too big. If you go with 20s you'll need rubber bands for tires and that means a harsher ride, plus you have to lower it to make it look good. If you do have your heart set on 20s, there are people who have done it, especially in the sedan (W204) forums so they can help you, too.

I agree on the opinions above regarding appropriate size/s of wheels for the C Coupe, but it's always a case of each to their own so if it's 20's you want go for it

From research I've done on wheel specs for the C Coupe those specs above will present a problem on a lowered C Coupe or even a W204 sedan.

Depending on the drop, the problem may be worse if lowered a lot especially at the rear to make it work you'll have to do what I've seen on many C Coupes with large rims and wide tyres with low offsets and that is camber them or fit 'stretched tyres' so maybe the 275's won't work, both those options is certainly something I wouldn't do.

All the wheels I've seen on C Coupes or sedans that are of 19" or 20" that seem to work have offsets of ET 45 or greater in fact the examples I've seen of wheels that 'fit in' nicely have offsets of 50 and above, hope that helps

~ the offset (et 40) stated is too low for this car. the tires would need to be stretched having the rim stick out past the tire... not the look for a refined mb car imo.

Thanks guys, so what's a better offset with a 20x8.5 and 20x10 wheel? 47-50?

I think the set wheel offset is +48 front and +54 rear. You can vary from those but have to be careful of the width, especially if you lower.

As already noted above be very careful with rim widths, 10" wide for the rear is pushing limits IMO adopt no more than 9 or max. 9.5" width.

You'll still need to stick to an ET of no less than 45 regardless to maintain a healthy clearance, so 47 and/or 50 is good as this offset will keep the edges of the rims within the guards and not poking out personally I'd take the 50 offset.

With a 10" wide rim at the rear and appropriate offset you'll have to check that it's not too close to the inside of the wheel arch tub.