Lots of good comments in this thread already. A couple of things I would like to add:

Although I mentioned the Yoko AVS, I have to admit the last street tires I bought for the '67S were Bridgestone RE71s. They are older tech, discontinued tires now, and hard to find in some sizes. They are only 140 treadwear rating (which is relative only to other Bridgestones), and aren't the longest lasting tire, but they are great to run in autox in a "street stock" class if you aren't allowed to use R-compound rubber, and only $56 at Tire Rack.

I purposely bought 195-50s, a very short tire (only a little more than 23" tall), because of the gearing advantage for autox and the fact that it lowers the car by about 1/2". The section and tread width on these is as wide as my 205/60-15 Dunlops, which previously cost me 2 points in "extra width handicap" in our zone. There can be a lot of variation in actual dimensions amoung different manufacturers, so check the specs carefully. The 245/45-16 Kumho is a full 1/2" thinner in section than the identical size designation 245 Hoosier, for instance. You would think that the difference in width between a 265 Kumho and a 275 Hoosier would be about 10mm, right? It is actually almost an inch larger on both tread and section dimensions, while remaining close to the same height! It seems to me that this "fast-and-loose" sizing policy by the tire companies could render all your estimating with a tire-size spreadsheet moot, in the same way that their treadwear ratings are not standardized across the industry.

As far as your speedo accuracy goes, be sure and check how it is calibrated now, before you change sizes. I was sure that with these new tiny tires my freeway indicated speeds would be way off. I put the RE-71s on and took my GPS with me on the drive to Dunkel's this weekend, and although I was turning 4250 RPMs at 75 mph indicated now, I was very surprised to see the GPS registering 74.9 mph! Multiple tests at different speed ranges with good satellite fixes showed that my speedo was actually perfectly accurate now, more so than before. It was under-reporting speeds previously!

As a carpenter, I offer a saying that was handed down to me by a master who was training me in the trade 30 years ago: "Measure twice and cut once!"

Good driving to you all,
TT