Gearing and Performance Question - Cup2 versus Pilot SS

I'm hoping that some of you engineering-types here can help me understand the math when it comes to the gearing difference between the 30-series Cup2s on the R and the 35-series Pilot SS.

I'm starting from the assumption that gearing in the differential and transmission are the same for R and the other versions of the GT350, so please correct me if I'm wrong on that. Even so, if there are differences in the trans and rear end, that just makes for more calculations, though, as there still may be a gearing advantage to the R, which is what I suspect is one of the main drivers of performance differential.

I'll stipulate to the fact that the R has the advantage in many other areas, including wheel and tire weight, overall vehicle weight, spring rates and weight transfer, stability control differences, etc. So, before we fall into the argument on all those points, let's talk about gearing and the effect that smaller tire diameters have on the R's ability to accelerate faster.

We'll need to know the ratio for each gear, as well as the differential, so hopefully someone has that info to supply.

According to Tirerack.com, the specs for these two tires in OEM form are:

Cup2:

Overall diameter -

305/30ZR19 = 26.3"
315/30ZR19 = 26.5"

Revolutions per mile -

305/30ZR19 = 792
315/30ZR19 = 785

Pilot SS:

Overall diameter -

295/35ZR19 = 27.1"
305/35ZR19 = 27.4"

Revolutions per mile -

295/35ZR19 = 767
305/35ZR19 = 758

As mentioned above, my guess is that if the gearing in the trans and differential are the same for the R and non-R cars, then the smaller tires are going to give a mechanical advantage in acceleration to the R, with a corresponding drop in overall top-speed potential.

Thoughts?

I should also mention that I have a set of the 6GR wheels with the Cup2 tires in OEM sizes that I'm going to be using for track days, so my interest in this is to determine what, if any, mechanical advantage I'll experience with those due solely to the size difference.

Yessir, there will be a slight "gearing" advantage for the R simply based on overall diameter of the wheel/tire. Not only will it benefit slightly in acceleration, but it will also help in deceleration as well, for the exact same reasons, with weight being slightly more centralized around the axis of rotation.

Combined with the large reduction in weight from both the wheels and the tires, it is why you see almost a 0.5 second advantage in testing for the 60-130mph run as tested by CarandDriver, with a significant 1.3 seconds advantage in the pull to 150mph. Keep in mind too that the R has significantly more downforce as well, so in reality the difference would be even greater were a non-R GT350 swap out to 30 series tires along with similarly lightweight wheels for example.

And you are correct, this is why the R tops out around I believe 173mph whereas the non-R can approach 178-180, as 6th gear is simply too tall for the car to overcome aero forces at those speeds.

The gearing is indeed the same. Because the Cup 2s are smaller in diameter, they transmit more force to the road; using your numbers: 27.4/26.5 = 1.034, the Cup 2s put down 3.4% more force under equivalent conditions (gear, RPM, throttle position, etc.). This is purely the mechanical advantage of the tire diameter. There are potentially additional advantages due to reduced inertia of the Cup 2s (assuming this is the case). Certainly the Carbon Fiber have reduced inertia.

I have done the 6GR wheels with the 325/30-19's on the rear (I realize there is less diameter difference to the 305/35's) and I don't notice any difference in acceleration that would warrant an analysis. I think the variances in the mechanicals and components and the vehicle weight between the R and the GT350 are a significantly greater factor in the performance difference than the tire diameter.
Here's a video that should answer most if not all of your questions, if you need to pursue the calculations on the impact of tire size and acceleration.

[ame]

I suppose it would be worth noting that the stock non-R tires (Michelin MPSS) will be replaced with the new MP4S tires. According to listings on tire rack, the new 4S tires are only offered in 30% sidewall (I could be wrong but this is all that is listed). Just FYI

Chris,
Check the front few pages of your Supplemental Manual. The gearing info you're looking for is there. I tried to cut and paste from my phine but it didn't line up....

And this is also another reason I wish we could get 4.56 gears for this car.