I know this might be a bit **** but I have always assumed that the best looking wheel/tire sizes are those which are pretty much centered in the wheel-well and therefore lowering the car should have this position as the ideal [I am thinking about purchasing VAP lowering springs (22mm drop) for my 18" stock wheels and stock-sized tires]. I am not interested in upgrading to 19" or 20" wheels/tires.

Any thoughts?

 

I have the VAP springs with stock 20" wheels and stock size tyres and using my Mark 1 Eyeball Guessometer I reckon the tyres sit just a smidge higher than centred in the wheel wells, maybe about 2 mm. In which case a drop of 20 mm would be "perfect" for you. This is based on my assumption that the overall diameter of the stock 18" set up is the same as the stock 20" set up, both front and rear.

 

Are you assuming that the 18" and 20" wheels/tires have the same overall diameter for speed calibration purposes? And my compliments on your ability to judge so precisely [especially considering that there are several variables involved, e.g., the surface which flattens the inferior aspect of the tire]. If it wouldn't be inconvenient, could you measure the distance between the top of the tire and the bottom of the visible wheel-well front and back? Also the overall diameter of your 20" tires? Thanks!

 

Not necessarily for speed calibration purposes but mainly because as the tyre diameter goes up the aspect ratio goes down by much the same ratio, so as to preserve much the same overall diameter.
And thanks for the compliment although I doubt my Mark 1 Eyeball Guessometer is quite so precise especially seeing as in my dotage my eyesight is nowhere near as sharp as it once was!
Anyway some rough measurements using a steel rule for the gaps and a tape rule for the diameters, again not very precise and again using my Mark 1 Eyeball:
Tyre to fender gap front and rear both 30 mm.
Tyre diameter front 670 mm rear 685 mm.

 

I have been following this coz I'm interested, a fun fact - my tyre diameter (21's) is the same. Hard to measure exactly coz of the curve of the tyre.

 

Finally got out to the garage to measure and indeed mine were 670mm front and rear. Measured the rears twice. Makes sense that the tire diameter needs to be the same and suppose this is how the aspect ratios are chosen for different wheel sizes. That's for checking that out! Btw, the gap measures 38mm front and 41mm rear [before lowering].