No secret, it's a General Tire Altima XP, which weighs 18 lbs in the 205/55/16 size. I'm sure a lot of sidewall stiffness is sacrificed but with the -20lbs and about 60 psi I'm hoping for about 2 mpg more. A high performance tire would be in the 24/25 lb. range. Your car, of course has lighter and smaller tyres but every bit helps.
By the way, an EP3 Si over here looks the same but is quite different from the UKDM version (CTR).
General Altimax HP