Part 2 of previous Post / Related, but new topic:
(This is still Rob in SoCal talking)
This also go me thinking about rim width. Why does one guy run a 275 on a 9" wheel and the Nissan guys run one on an 11"?
From all my reading, to a point,not always, wider is better for rim width if it is within the tire manufacturers recommendations. Look at Porsche they regularly spec tires on Wider than recommended wheel widths from the factory. My goal was the flattest tread profile for whatever tire size I choose. In reading every .5" of added rim with will add .2" of measured tread width on the same size tire. I looked to match the wheel width and tread with dimension for my chosen tire heights.
Some tire & wheel basics: Whatever tire width you put on whatever rim width … there is a “happy window” of tire pressure to achieve a full & even tire contact patch on the pavement. Every tire sidewall acts like a spring and the spring rate is affected by tire pressure. More pressure = higher spring rate. All tires do weird things when you get them out of their optimum spring rate. Until just recently I had & utilized an Intercomp tire sidewall spring rater to test sidewall spring rates of different tire & rim combinations to find their “happy window” & to know at what point they got weird.
When you have the tread width wider than the rim width, you have to run lower tire pressure to achieve a full & even tire contact patch on the pavement ... otherwise you’re “crowning” the tire & using less than the full tread width. This lower tire pressure makes the tire sidewall spring rate “softer.” The softer sidewall COMBINED with the fat tire sidewall bulge, leads to the tire “moving around” on the rim substantially. This is ugly when the car is pushed hard, like in track days or AutoX, but provides a softer ride.
When you have the tread width narrower than the rim width, you have to run higher tire pressure to achieve a full & even tire contact patch on the pavement ... otherwise the tire tread goes “concave” … not utilizing the center tread … ending up with less dyanamic tread width. This additional tire pressure makes the tire sidewall spring rate “stiffer.” The stiffer sidewall COMBINED with little to no tire sidewall bulge, makes the tire substantially “more stable” on the rim. This combination performs best … BY FAR … when the car is pushed hard, like in track days or AutoX, but provides a harsh ride.
I wanted a 26" tall front and 27" tall rear tire to fill my wheel wells and maximize the length of the contact patch and the tires ability to radiate heat. Out back that was a no brainer. 345 30 19 on a 13" wide wheel. This is the Dodge Viper spec. Michelin built that tire originally for the Dodge Viper. Am I smarter than those companies engineers? No.
Not to be a smart ass … ok … being a little bit of a smart ass … is your car a Dodge Viper … with the same suspension set-up & geometry? No. 
In my opinion, I always suggest when you’re copying something, be clear on why.
So what to match it with up front. Well a front engine rear wheel drive 3400lbs Viper uses a 275/345 combo with a 50/50 weight distribution. I may not get to 50/50 but I'm trying to get as close as possible.
That will be awesome. The closer you get the weight balanced front to rear … and side to side … WITHOUT placing weigh out past either axle to get it … will be beneficial. Get as close as you can.
I'm also hoping to come in around that weight. Before anyone else says it I know there are more factors than that. To start with CG to Roll center creates a moment arm which actually influences load on the tire as much as total weight. Like I said before I'm not building a Rolex competitive racer so I'll keep it simple.
Sounds smart. Working with track width, tire & rim width are simple, predicatable tools.
I borrowed a 275 35 18 mounted on a wheel from Ron, thanks again Ron and a 295 35 18 tire from Brett, thanks bud. the 275 was an easy fit and the 295 ,using my wheel fit tool, was a little big. A 285 should be just right I thought.
Here is where Travis really came through. I had communicated with him a few months ago about a quote for my wheels. I really like the fact his wheels were clean timeless designs and engineered for and used in racing. From my wheelfit measurements I was pretty certain I had a BS measurement and wanted a 10.5" wide wheel wheel.
The 10.5" wheel did two things. The measured tread width of a 285 Mich on a 10" wheel was 10.2" So adding .2" for a .5" increase in wheel width this gave me 10.4" tread width on a 10.5" wheel width. A good match in my mind. It also gave me a 2" outside lip with my backspace measurement to avoid Jiffypop.
I really need to expand my terminology. I’m behind. Does “Jiffypop” mean the wheel center is crowned out … or flush with the outer edge of the wheel … or both?
In speaking with Travis he said he could build a test wheel to the same specs I thought I needed actually try it on the car. Hell yea. A week alter I had a borrowed wheel made from both new , blem and used parts. Travis sealed it so I could mount and air up my tire.
This was a huge help. I was able to confirm my thinking on backspace, the profile of the 285 on the 10.5 matching that of the rear tire, double checking caliper clearance and getting a real world visual can't be topped.
Tires are disposable items. My car won't be painted before I actually start tuning the set up. I plan on running it in shake down form for a while before I really finish it pretty. If I decide to go wider and do more tweaking on the body it won't be big deal. The 10.5" lets me use a 295 or 305 and still get a good footprint if in the end I feel the 285 is a limiting factor.
My suggestion would be … if anything I wrote influences you to go to wider front tires … do it now. Do it once & do it right, by getting wheels that are a little wider than the tread width. But if you’re happy with the 285’s … party on Garth.
I still think the 285 on the 10.5" wheel will give me all the front bite I need if my shocks, springs and bars are tuned correctly.
This will be your limiting factor in performance driving. If you really dial in the front suspension … use enough caster to end up with a KPI/Caster split favoring the caster by 1.0+ degrees, moderate camber gain, -1.0 camber, correct toe-out/Ackerman balance, zero bump steer & the optimum roll center … plus the optimum sway bar, spring & shock combo … the car can go no faster in the corner than the 285’s can grip. So you will be reducing the grip of the rear end to match the 285’s. As far as the performance is concerned, you might as well run 285’s on the rear, because all the tuning you do, will be to reduce the grip of the tires to the level of 285’s.
Now I’m going to take a different position.
This isn’t a race car. It’s your car. It’s your car to drive & have fun in however you want it to be. That may include track days, competitions, spirited driving, etc. I think you should build it the way you want it, which is what I love about Hot Rodding in general & Pro Touring in particular. I get the look you’re going after … because I like it too.
For looks: I also prefer a small 1.5-2” lip on the front wheels & a deeper dish (lip) on the rear wheels.
For handling: I prefer to get the scrub radius down to zero or close to it, which requires the face of the wheel to be moved out close to flush, kind of like the SCCA GT1/Pro Trans Am cars are. For me, handling always ends up winning on the priority list.
I attached a photo of my "Street tires & wheels" from American Racing. I think they look ok. But they "Jiffypop" so I could achieve a zero scrub radius front suspension.
In the photo:
On the Left, are my flush face front wheels: 18x10" AR "Burnout" VN472 Wheel with 8" backspacing. Tires are Nitto NT555 285/35ZR18
On the Right are my deep dish rear wheels: 20x12 AR "Burnout" VN472 Wheel with 4" backspacing. Tires are M/T S/R 29X15.00R20 = 365/30/20
* For Autocross & Track Driving these tires & wheels come off & "front & rear matching size" tires & wheels go on.
BFG Rivals 335/18/30 on 18x13” CCW lightweight LM5 rims.
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