I was looking for something else and ran across this Car & Driver article about the effects of "upsizing" wheels and tires.

As expected, extra weight and rotational inertia have a negative impact on acceleration and fuel economy, but better cornering on the skid pad. I was surprised by the difference in acceleration.

Of course, the extra grip provided by larger tires offsets some of the disadvantages, not to mention that they "look" nice.

The test could have been run better and I see a few issues, but overall it seems to make sense.

Effects of Upsized Wheels and Tires Tested - Tech Dept. - Car and Driver

And there's the cost of the wheels and tires, but it's what you can afford.

Better if they ran some tests with the same rim diameter and then went wider but I understand why they ran the tests like they did.

Of course changing the diff just might have made those plus size tires perform just as well if not better.

I used to time trial my Nissan 350Z with NASA in their TTC class. At California Speedway the car was 1 second per lap faster on 245/45-18s then on 285-30-18s. I figured it was because the car spent so much more time over 100mph on that track. Conversely, at Buttonwillow the car was about 1 second per lap faster on the 285s. Tighter track.

John - excellent point. I noticed a different when we run the 15X7 rears at a vintage event and then the 15X10s at other events.

Pete and I are building a second engine for the 240 so we can run the 2.4L on tight tracks and the 2.8L on the longer ones where we would go out in a session with the V-8s instead of the 2.5L and under crowd.

I think the lap time issue at Cal Speedway had more to do with aero then grip or non-grip. The front 285s stuck out a bit past the front valance and probably added some drag over 100mph.

When I asked Bob Sharp which of all his Zs which would he chose to drive if he could only have one and he answered without hesitation, "the Z with the 15-inch wide rears" and he followed up with "give me 30-inches of rubber and a soft suspension and I can put the car anywhere I want it to go".

The most we can use in vintage are 225s on 7-inch wide rims. Would love to have those 285s you ran.