The title says it all really. I finally found some used 16" Skoda Spectrum alloys (they're fitted as standard to Greenline models) to replace my Superb II Elegance's original 18" Themisto wheels.

 

This is the swap I am making:

 

205/55 R16  94V  7J x 16  ET 45
225/40 R18 92V 7.5J x 18 ET 46

 

Sorry for being really thick but will I need to buy and fit spacers to keep the 205-wide Spectrum alloys relatively in line with the arches, or will they look fine as Spectrums are authentic Skoda Superb wheels?

Skoda is known to have "hidden wheels", whether its 18's or 16's. The difference between the two wheels is only 5.4 mm, so I don't think you will see any big difference. Some, but not much. It will be more noticeable that the profile height has increased. Personally I would never fitted spacers to my car.

You don't need spacers but they can be added. I have 15mm spacers on the front of mine and 20mm on the rear. It makes a big difference to the look of the car. It's far more Germanic now as the wheels look like they fit the car as they should, in my opinion at least!

No need to add any spacers. Spectrum are available for Superb as accessory option and you can fit them with standard bolts

I've run two Yetis on the these wheels (for my winter tyre setup) - no problems whatsoever without spacers. Sorry to drop in so late with this comment - I've been out of action due to health problems.

I'd fit 225/50 16s on those rims but let the tyre achieve the wider look instead and avoid the need/urge to fit spacers.

Slightly off topic, but don't spacers add additional strain on the wheel bearings?

^^^ 

Supposedly, but seeing as how many world wide use 'good' correctly fitted / tightened wheel spacers without issues, including in Motorsports (Aprroved Max Width in MSA Regs etc)

If you do not fancy them just 'Say no',  or be aware of possible issues, and be sure the bearings are good before fitting spacers, and see how they last & if the life is shortened in any way.