As far as performance goes I'm not sure what the difference is but the Nero's cost quite a bit less then the Rosso's. Hear is what Tire Rack has to say about them:
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The PZero Rosso System includes both asymmetric and directional tread designs, each tuned to satisfy the unique performance needs of different types of vehicles and uses. The PZero Rosso Asimmetrico is the most widely used because it's designed for both front and rear axle positions of front engine front-wheel drive and rear-wheel drive cars (and on all cars driven on the track). The PZero Rosso Direzionale is designed to offer enhanced hydroplaning resistance and is primarily for use on the lighter load carrying front axle positions of mid-engine and rear-engine rear-wheel drive cars (which are then combined with the PZero Rosso Asimmetrico tires on the rear axle positions).
Both of the PZero Rosso tread patterns are elegant. The Asimmetrico tread pattern retains the "Zero concept" of earlier PZero tires by combining a semi-slick center rib that enhances driving precision while accelerating and braking with robust outer shoulder and intermediate tread blocks that are connected by circumferential "multi-ring" ribs to enhance dry grip. The independent blocks of the inner intermediate rib and inside shoulder are separated by wide grooves to help eject water to reduce hydroplaning and enhance wet grip. Considerable use has been made of curved grooves, which complement the circumferential grooves to reduce noise.
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The PZero Nero (ne' ro' Italian for black) Max Performance "summer" tire is a member of Pirelli's "PZero Collection" family of Max Performance tires. In addition to the PZero Nero, the "PZero Collection" also includes the PZero Rosso (ro' so' Italian for red) and the PZero System Directionale and Asimmetrico tires (many with a yellow logo) for sports cars, coupes and sedans. The PZero Nero was developed to work harder so the driver could play harder exploiting the sporting character of their high performance car. Like all summer tires, PZero Nero tires are not intended to be driven in near-freezing temperatures, through snow or on ice.
On the outside, the PZero Nero features a silica and carbon black enhanced tread compound molded into a subtle asymmetric, wide rib tread design to increase the rigidity of the tread to resist deformation and enhance grip and steering response on a variety of road surfaces while enhancing resistance to irregular wear and generating low noise levels. The tire's circumferential grooves are optimized in their position and width to maximize dry road performance while enhancing hydroplaningresistance. On the inside, the PZero Nero features twin steel belts reinforced by zero degree nylon to complement a lightweight aramid internal structure that resists deformation at high speed
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I have PZero Rosso's on my Gallardo. They grip well but I must say they don't wair very well. I perfer Michelin Pilot Sports.
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