Had another learning session last night with Mark Savitske at SC&C mostly on things NOT to do. See my earlier posts about my previous learning sessions from him. As a reminder he has spent decades working with 68-72 GM A-body cars and making them handle better for street, track etc. I'm fully drinking the cool aide now and have most of his parts/suggestions on my 442. He also wrote the book “How to Make Your Muscle Car Handle”. Anyway my plan was to upgrade wheels and tires to a combo of taller wheels and tires. Was leaning towards 20x11s in the rear on 305/35/20 Nitto 555rs and either 20s or 19s in the front in a 10" wide or maybe even 11" wheel (if it could be made to fit) with a higher end high performance front tire. I was really liking the "square" setup look with 305s front and rear for example. I've seen a few cars done like this and the new muscle cars like the Hellcat and GT350 Mustangs are running similar sizes.
After talking with him it turns out this would be counter productive with the other parts I've already added that currently work really with the roll center of the factory GM A body frame. Mark confirmed ideally the rear tire should be at or around 27.5" tall and the front should be at or around 26.7" tall. Since I'm currently at 28.78/27 I'm too tall now and didn't realize I could make things worse by going taller. Also Mark explained with my setup the wider front wheel/tire combo could actually make things worse and cause issues with turning radius and other things that were working really well as I have them now. He explained going with a 305 front and rear square set up could work but I would need to make other changes to accommodate (tear out stock inner fender, add wide body flares, because it makes the wheel turning radius even wider - have to alter several things - there's a long list here ). Additionally going that wide could cause issues for street driving including simple things like making a U turn, parking etc, plus could cause trammeling on the highway where the front wheels track left and right on imperfections on the road. And then there's the additional weight. Bottom line he suggested staying with an 8" wide wheel front or a max of 9" and suggested only a
245 45 18 front tire - better to stretch it on a 9" wheel vs the
255 45 18 I'm running now on an 8" wheel. He said he's had several clients win and/or do very well running only a 245/45/18 tire at autocross events and even put down faster times than those with much wider setup for the reasons explained above especially when there are 180 degree turns in the autocross event. For the rear he said an 18x10 with something like a 285/45/18 is all you need. I've currently got a 305/45/18 drag radial on an 18x10 which I explained had lots of room all around due to clearancing in the wheel well, rolling the outer lip, and flat spots put on the 3" tail pipes. He said that was great but told me he bets if I put it though an autocross event it would hit. We'll see but apparently my current tires are not ideal at 28.78" in the rear. Nitto 555r does make a 305/40/18 drag radial that is only 27.7" tall and he said that would be great for height but would need to be on an 11" wide wheel (10" would not be ideal) and if it didn't hit (again TBD) would be excellent for autocross at about 40 psi. He confirmed he's had several customers running Nitto 555rs successfully in autocross however he did forewarn that many autocross events require treadwear rating of the tire to be 200 or above to have your times "count" if you're trying to place. You can still do the event for fun and get a time but it won't count as far as placing. The Nitto 555rs are 100 TW rating.
So in summary my plan is to swap out my front Nitto
555 255/45/18s for some 245/45/18s and might even upgrade to a 9" wheel at some point. I found the Mich Pilot 4S tire would actually be 4.8" lbs lighter than the Nittos. That alone would be a huge improvement to drop almost 5 lbs of unsprung weight on both sides. Then for the rear I may upgrade to a 18x11 rim and drop down to 305/40/18 555rs. If they rub during autocross I can use them for street and drag race only and then buy a set of 285/45/18 tires to go on my current 18x10s. These could also be used if the 100 TW rating of the Nittos becomes an issue for certain autocross events.