Comment on my setup before I buy? 66 A-body with staggered wheel diameter

[ignaro]

Hello all!

I have a full UMI suspension kit (http://umiperformance.com/catalog/index.php?main_page=product_info&cPath=1_253_335&products_id=717) and will be ordering 325mm disc brakes from Kore3 (http://www.kore3.com/proddetail.php?prod=10103-01) along with these wheels and tires. I've measured everything and am about to order. I'd love to get any and all thoughts!

Currently I have 225/70R14's all around. Those are 26.5" tall and 9" wide.
My new front tires will be Michelin PSS 275/40R18 (https://www.tirerack.com/tires/tires.jsp?tireMake=Michelin&tireModel=Pilot+Super+Sport&frontTire=74YR8PSS&rearTire=74YR9PSSXL&vehicleSearch=false&fromCompare1=yes) which is 26.7" tall and 10.8" wide, roughly the same height.
My new back tires will be Michelin PSS 275/40R19 (https://www.tirerack.com/tires/tires.jsp?tireMake=Michelin&tireModel=Pilot+Super+Sport&frontTire=74YR8PSS&rearTire=74YR9PSSXL&vehicleSearch=false&fromCompare1=yes). These are 27.7" tall and 10.8" wide. This should fill out the rear wheel well better, as pointed out in other threads. It's also much less drastic than I'd been thinking before.

Wheels will be 5x120 on my 5x120.65 bolt pattern. We agree that's cool? (https://www.thirdgen.org/forums/wheels-tires/738077-5x4-75-vs-5x120.html)The tire store hates the idea but they wouldn't want to be liable.
The center bore is 72.6mm which will need hub centric rings (https://www.walmart.com/ip/Hubcentric-Rings-Pack-4-70-3mm-ID-72-6mm-OD-Black-Poly-Carbon-Plastic-Hubrings-Hub-Works-70-3mm-Vehicle-Hubs-72-6mm-Wheel-Precision-European-Motorwer/322612569?wmlspartner=wlpa&selectedSellerId=15374&adid=22222222227117178748&wl0=&wl1=g&wl2=c&wl3=233402377367&wl4=pla-576520567632&wl5=9031008&wl6=&wl7=&wl8=&wl9=pla&wl10=118787069&wl11=online&wl12=322612569&wl13=&veh=sem) down to 70.3.
The wheels are Niche Misano's in gray.
Front is 18x9.5 with +40mm offset (https://www.amazon.com/Niche-Misano-18x9-5-5x120-Anthracite/dp/B00U6YU4WE/ref=sr_1_cc_1?s=aps&ie=UTF8&qid=1512511241&sr=1-1-catcorr&keywords=5x120+misano+anthracite). The Kore3 brake kit adds +3-5mm track width, so I'm thinking of the wheels as being +35 or so.
Rear is 19x9.5 with +35mm (https://www.amazon.com/Niche-Misano-19x9-5-5x120-Anthracite/dp/B00U6YNRL4/ref=sr_1_5?s=automotive&ie=UTF8&qid=1512511241&sr=1-5&keywords=5x120+misano+anthracite) offset. Not sure why that wheel is different but regardless, that should be good.

I'm going to remeasure tomorrow but I believe my wheels are 0 offset. It's hard to measure accurately with so much tire bulge.
According to https://tiresize.com/wheel-offset-calculator/ the outside of the wheel matches up at +24mm offset for that tire.

I plan to order the wheels, tires, and rings first. Then I can order the 10mm spacers I'll likely need once I confirm that's correct.
I'm a slow shade-tree mechanic so it'll take me a bit to do the brake swap, plenty of time to make sure I get exactly the spacers I need.

Any thoughts or criticism is welcome! Hard to return any of this stuff when you're buying it piecemeal! Thanks!
146696

[USAZR1]

A 9.5" width wheel with +40 offset = approx. 7" backspacing
A 9.5" width wheel with +35 offset = approx. 6.8" backspacing
This should be interesting.

[ignaro]

A 9.5" width wheel with +40 offset = approx. 7" backspacing
A 9.5" width wheel with +35 offset = approx. 6.8" backspacing
This should be interesting.

Sarcasm aside, thanks for replying.

(9.5"+1")/2=5.25"+1.57"=6.82", or +1.38"=6.63", that's not far off threads I've seen like this one (https://www.pro-touring.com/threads/101081-Fitting-275-40R18s-on-18x9-5-quot-wheels-on-all-four-corners-of-a-66-Chevelle?p=1012368#post1012368).

Would I be better off with 8 and 8.5" wide wheels? Same offset? This chart (https://www.budnik.com/helpful-tire-info) says 9.5 is on the low end for 275 tires.

I'm largely relying on that tiresize.com calculator. I do have to double-check that my current wheels are 0 offset, they may have a small offset that I missed initially.
Regardless, I'll obviously need a small spacer to get them just right. If my guess/measurements are correct, it should be a 10mm spacer like this one (https://www.amazon.com/Hubcentric-Spacers-5X4-75-5X120-65-5X120-7/dp/B06XNL535V).

Do you have any suggestions for a better fit? I wasn't able to find a wheel around 18x9 with +25 to +35 offset in a 5x120.65 bolt pattern, let alone one with a matching 19" version and that looked nice. My setup certainly relies on more adapter stuff than I'd like-- any improvements would be greatly appreciated!

[USAZR1]

No sarcasm. Why don't you ask Tom Farrington why his front wheels have so much backspace, over an inch more that is normally suggested for that width, on these cars? The other guy(s) in that thread were going to use 5.5" backspace on a 9.5" width front wheel. (which is the norm)

Next question: Did you wonder why you can't find 18x9's with +25 to +35 offset in a 5x120.65 bolt pattern, unless you custom order them? That's because a wheel with those specs won't fit most of these cars, unless they have considerable mods that might require it.

Last but not least, you are guessing what will fit because you haven't measured your car. When you're trying to fit big rolling stock under these older cars, you can't guess, unless you have a very good handle on what your trying to accomplish. Another thing, A-bodies are not all the same. Your 66 Tempest may not use the same wheel specs, as a 66 Chevelle/Elco. My guess, and i've been doing this for awhile, is a wheel with either +6 or +12 offset will probably fit your car very well. However, I would find the guys on this site with 65-67 Lemans and GTO's and pick their brain, to make sure. Andrewb70 can help too, I bet. Failing that, I would buy a wheel backspacing tool from either Jegs or Summit. Those aren't very expensive and could save you from purchasing the wrong wheels.

I apologize if I sounded harsh, as that's certainly not my intent. Just want to help.

edit: Here's a thread to look over. https://www.pro-touring.com/threads/51788-67-GTO-maximum-wheel-tire-sizes?highlight=Pontiac+GTO

[ignaro]

That's exactly the thread I couldn't find before, thanks! I had seen it once and then my searches didn't find it.

I had measured before and couldn't figure out why my numbers were coming up different, hence the post. I took the day off work today and re-measured everything and came up with exactly +35 offset needed on the 9.5" wheel and 275 tire.
For posterity, here are my measurements:
My current wheels are 14x7 with +3/8" offset which is about 10mm. The 70 series tires bulge by 3/4" in front and 5/8" in back. The whole thing is 9 3/8" wide. I have 3/4" clearance outside the wheel to the rear wheelwell.
From the wheel mounting surface, there's 4 3/4" of 'frontspacing' and 7.5" of 'backspacing' room. 12.25" total in the wheel well.
If I'm fitting an 11" wide tire in there, it should have 5/8" on either side. That means the wheel needs 1 3/8" of offset, or +35mm. Shazaam!

In the front, I have 2 7/8" before hitting the sway bar at full lock on the left and 3 1/4 on the right... I blame the rebuilt steering box.
The Kore3 brake kit adds 5mm track width, effectively making my +40 wheel a +35.
At that offset, the wheel goes in 2", leaving me with about 1" before the sway bar rubs.

The one measurement I didn't have right in my first post is that my current wheels have a +10mm offset already in them. Had I bought +25 offset wheels, those wouldn't have fit.
Truly had to measure twice-- now I've bought once. Here's hoping everything lines up correctly!

Clint-- thanks again for your reply!