Just a thought that's been running through my mind.

Do tyre manufacturers make specific tyres for new cars that have slightly less tread than those sold as standard retail?

Yes they do fact! ABC told me this, somewhere the sidewall codes show this 

America has UTQG on sidewall too

 

The supplied tread depth is only half the story of how long a tyre will last. The compound composition, rolling resistance and hardness of rubber all have a huge influence. Anyone who rides motorbikeikes will tell you this.

Interesting. My Nokian Z G2 summers came with 8mm of tread - it's easy to tell as they have the 8-7-6-5-4-3-2 etc marked on the tyre as wear indicators. Not sure why nobody else does that. The reason I mention the Z G2's is that they are intended to be a direct competitor to the Conti SportContact5 (they even look pretty much the same). They're nice tyres, great feel and excellent stopping power. I'd run them over Uniroyal RS2's, definitely. 

CSC5 come with 9/32" tread (7.1mm).  Depending on size, CSC5P come withe either 9/32 or 10/32" (7.9mm).

 

CSC3 mostly came in 10/32" except for a few OEM spec porsche/BMW specific sizes.

 

hopefully that 10% less tread won't translate into 10% less mileage