Before you decide to FLAME me, I did a search and found a few threads that gave me a good idea of what was available, but no one seemed to have an idea of what WORKED best. Here are the specs:

'02 Cooper S lowered about 1 inch all around with H&R Springs, wheels are 18X7.5 with 42ET. Would 215/40/18s serve me well? I don't expect the ride quality to improve, but is this a good size tire to maximize protection against possible bends? Thanks for helping!

ron.

 

40/215....perhaps

 

I think that you have to look at the tire specs for each brand and size of tire individually. They all differ some what. A good place to start checking is at Tire Rack.
Steve

 

I agree with what THE ITCH said, all tires differ in size. Example, I bought 205/40/17 Zeix 912 and they're basically the same width and overall diameter as the Falken 452's in a 215/40/17.

BUT, with H-Sports and that offset I'd go with 215/40/18 cause it'll fill out the well a little nicer than the 35 will.

 

Your lowered MINI has less clearance.

215/40-18 would be better with stock suspension since it is taller than stock tire diameter at 24.7". It fits rims 7-8.5" wide.

Other sizes that would work with 18x7.5" rims for lowered MINI-
215/35-18 fits rims 7-8.5" wide, perfect for 7.5" wide rim, 23.9" tire diameter
225/35-18 fits rims 7.5-9" wide, perfect for 8" wide rim, 24.3" tire diameter
(This size tends to be lighter in weight).

 

I had 215/35/18 on 18x7.5 SSR Comps, ET 42, and had zero rubbing during normal driving. Plenty of clearance both inside and out. I would've definitely preferred the slightly smoother ride of the 40 series plus the overall look. I only rubbed on BIG dips in the road at speed and with a full car, which was rare.

 

Just keep in mind your clearance.

By comparison I had:
1 inch drop from H-sport springs on an MCS using stock shocks
215/45-17 (both sets I tried) which is same 24.7" tire diameter as 215/40-18
on 17x7 rims (not even wider than stock) and +42 offset
I got rubbing in the rears- especially on any uneven roads any dips (with any sort of speed) and with any hard cornering, and with any passengers in the rear including one person 100 lbs and certainly with two even two kids.

There is some variation with each MINI and with some alignments, more negative in the rear will help with less risk for rubbing.

So ask yourself how smooth are your roads, how fast do you drive/corner, how much do you ever have more than one person in your car, and how much would occasional rubbing make a difference for you.

I think if you got lowering springs with Koni Yellow shocks it would be better than trying to use stock shocks which may not perform as well with lowering springs. Also the age of the stock shocks could be a factor, the older they are the more worn out they could be.