The diff operates so smoothly and quietly I have to try hard to get any chatter—like in a U-turn at part-throttle—yet it’s still fairly unobtrusive. With the stock diff, I’d gotten used to expecting so much useless torque that, with the traction control off, I’d take up two lanes when the tires broke loose. It was like driving in the rain. With the TC on, the car would hesitate annoyingly while trying to find traction in the first two gears. It was like being the passenger to a driver who gets on and off the throttle repeatedly.

With the Renntech/OS Giken diff installed, it’s an entirely different story. While you don’t hear it, you feel the improvement. Given the massive 700 lb-ft of torque, leaving the TC on during a First-gear launch is still the way to go. But the acceleration is so brutal, it’s as if the diff and tires have married themselves harmoniously with the ECU’s commands, allowing maybe 10 percent wheel slippage, and keeping the car catapulting forward through First gear with much less hesitation. The improvement tells the story, and a zero-to-60 mph rip at 3.7 seconds with a 12.0-second quarter-mile says it boldly.

Some have told me it feels like a roller coaster; other unsuspecting passengers become quiet from the literal loss of breath, similar to when I throw my two-year-old Alexa up in the air and don’t forget to catch her.

With the newfound grip and progressive locking feature in the differential, the performance is much more repeatable. It will be interesting to see how this new upgrade changes the car’s behavior in light snow expected in the upcoming months here in the Midwest.

In high-speed turns, the diff performs smooth as silk, so much so that one has to wonder why Mercedes didn’t use OS Giken rear ends for all of its twin-turbo V12s. With a $4,900 price tag, it’s probably no more than a $2-3k difference over the factory pumpkin. Given the compromise-free performance improvement, that 3% price hike over the total package of a V12 Biturbo Benz would be nothing to me if I had that kind of coin lying around for a new one today. But I don’t.

At least I saved enough to pony up the 9-year depreciated amount. And thanks to this upgrade, I’m now even gladder I did.

General Tire GMAX AS-03

I have to keep reminding myself the General Tire GMAX AS-03 is a true all-season tire, not because it doesn’t exceed my expectations but because I keep thinking I just drove an ultra-high-performance summer tire.

I’ve been to a few of these tire gigs, and the fact that General invited us to test its new tire at Infineon Raceway in Sonoma, Calif., was a bold statement. The track has myriad blind, bowled and off-camber corners, both uphill and downhill, some you take in Fourth gear. If cars (and drivers) are going to exceed their limits, it’s here.

The new AS-03 is General’s new ultra-high-performance (UHP) all-season tire designed to perform in almost any condition, from dry to wet and even in light snow. It was a good move since prior to 2008 the summer/all-season tires sales ratio was 60/40, and since then this ratio has reversed.

The V-shaped grooves on the GMAX offer optimum water channeling as well as an aggressive visual appeal, while tread block stability and stiffness enhances dry performance. Further in, cleverly placed sipes provide snow traction, packing the snow onto the tire like a snowball builds on itself.

This tire offers incredible new technology that the consumer will notice throughout its lifespan. First, GT’s new VAI (Vehicle Alignment Indicator) technology provides sipes on the shoulders of the tires that wear within the first few hundred miles if the car is misaligned, saving the tires from ruin later on.

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