Paired with the NT555Rs were a brace of Nitto's NT555 street-performance tires, sized 275/40ZR17. The standard-issue 555 has an overall look similar to that of its competition-bred "R" sibling, along with a treadwear figure of 300 and a more well-rounded overall performance envelope. If the idea of using the same set of tires for street and strip duty appeals to you, this combination of front 555s and rear 555Rs could be just what you're looking for.
As for driveability, the four-hour round trip revealed no new bad habits stemming from the differential swap. The Zip rear remained as smooth and noise-free as when we installed it, while the 3.54 gears put the car's 75-mph cruising rpm at a perfectly tolerable 2,400 rpm. A check of the trip computer at day's end, meanwhile, revealed an average-mpg figure of 20.0, which could easily have been improved upon by a less excitable driver.
Unfortunately, our track session proved something of an anticlimax. Given our previous best pass of 13.268, we were hopeful that the new gearset would, at a minimum, drop the car into the 13.1-second range. Instead, we could do no better than a 13.307 in the midsummer Florida heat, followed by a 13.321 before the sky opened up and cut short the day's activities.
Interestingly, the car did pick up more than 1 mph in trap speed (104.57, as opposed to our old record of 103.35), perhaps an indication that the lower axle ratio was putting the big LT1 more deeply into its power band at the top of the track.
Given that the C4 displayed no inclination whatsoever to spin the gummy Nittos off the starting line-and believe us, we tried-it's clear that either more power or a higher-stall torque converter will be needed at this point to make a significant impact on e.t.'s. We'll be addressing both in the near future, starting with a performance-oriented rebuild for the car's tired 4L60E trans. Stay tuned.