225/45-18

You fail to understand the critical parameters of selecting a tire.

One of these is "rolling radius" or equivalently "rolling diameter", the other is section-width ratio.
By selecting tires with the same rolling diameter, you leave the car at the same ride height and same roll axis.
By selecting tires with the same section-width you have the same grip ratio front-to-back. This preserves the oversteer/understeer relationship.

348 uses 215/50-17 wheels F and 255/45-17 wheels R
F355 uses 224/40-18 wheels F and 265/30-18 wheels R

rolling diameter is: width×aspect×2+wheel
215×0.50×2/25.4+17 = 25.46
255×0.45×2/25.4+17 = 26.03
255/214 = 1.186

225×0.40×2/25.4+18 = 25.08
265×0.40×2/25.4+18 = 26.34
265/225 = 1.177

So, if you were simply to put on F355 wheels and tires, you would "want" to lower the front by 0.17" and lower the rear by 0.15". The ratio of section widths is essentially unchanged such that you can control the oversteer/understeer relationship with tire pressure.

I have 275/40-18 tires on my F355 and lowered the rear by 0.15" to compensate and put the roll axis back where it wants to be..

 

I work in the tire industry and there is a bit of variability in the overall diameter of tires that are marked as the same identical size. In other words, A Goodyear 225/45/18 may be slightly shorter than a Michelin of the same exact size. It's all due to where the width measurement is taken across the tread. Most manufacturers provide the TRUE overall diameter of the tire on their spec sheets.

Mitch's example above is fine, but since modern tires are measured in metric, I personally prefer to not convert the overall diameter measurement in inches, but keep it in millimeters. And yes, I realize wheels are measured in imperial units. Don't get me started on TRX or PAX sizing...

All things being equal, here are the calculated diameters of the tires (stock 348 / stock 355 / alternative size proposed):
215/50/17 - 646.8 mm
255/45/17 - 661.3 mm

225/40/18 - 637.2 mm
265/40/18 - 669.2 mm

225/45/18 - 659.7 mm (this would fill your 348 wheel wells ~6 mm further in each direction from the hub center, as well as a 6 mm body height increase over the 348 stock size. When compared with the stock 355 wheel/tire, it's double that. You also have to consider tire to body and suspension clearance when changing tire sizes.

I have never heard of the 'grip ratio' mentioned above, however - I don't think it's a very useful metric to bother calculating. Keep in mind that Ferraris use narrower front tires to increase steering feel. A narrow front tire feels more 'alive' than a wide one. Furthermore, this 'grip ratio' doesn't take into account the sidewall height. It's akin to saying a 70's Corvette with 225/75/15 front and 275/60/15 tires has the exact same grip ratio as a 2015 Mustang with 225/60/18 and 275/45/18 tires just because the widths are the same. The taller sidewalls of the Corvette will roll over sooner than the Mustang, and the grip will be much higher. This is also ONLY considering the tire width - there are a dozen other factors that come into play with calculating 'grip'. It's not all about tire width.

 

Let me help you with your actual question you started with. Not some bull about nonsense that only matters to the garage and trailer queens trying to brag to their friends about numbers and figures and original factory correctness. Barf...

I have 18" speedlines 355 setup on my 348. The roads here are absolute **** so having more meat was pretty critical to me. I am running 225/45-R18 fronts 275/40-R18 rears.

What I do not notice = rolling radius, rolling diameter, grip ratio, oversteer/understeer bla bla bla.

What I do notice = great grip, great handling, comfort in knowing I am not going to taco my expensive wheels on some **** road pothole.

I did also roll my fender lips and tucked them way up so they dont get pulled down turning and hitting a bump or driveway. The rear wheel opening is pretty full of tire but I personally like the look.


Oh yea and they are FALKEN tires....... hahahahaha eat your hearts out purists


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Indy 500s? They freaking rule. There's no squeal...I'm still not sure where the limit is...

 

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Do you have some other kind of formula that keeps the ratio of front grip to rear grip rather equal ???
Without having to explain how air pressure alters tire grip;
without having to explain struts versus live axles versus unequal a-arms;
without having to explain suspension geometry and chassis roll;
without having to explain what happens when the roll axis is changed;
without having to explain how tire width and wheel widths alter suspension geometry ?????

I use this because it is simple.

It also fails to explain why different things happen (with respect to wheel width versus tire width) on a car with strut suspension and another on a car with unequal A-arm suspension. Especially when the car with struts has been lowered (and the pickup points not raised accordingly.)

But from the tire specifications alone (3 numbers only):: and a customer wanting wider front tires; how do you help him select tires where he won't go off the road backwards from getting too much oversteer ??

 

^

Good points, Mitch. Here's a guide my support team uses for my car.

Barry

 

Hahahaha..... Mitch... Dude man.... Chill out!



Fronts are 18x8.5 et36
Rears are 18x10 et55

And I was wrong, I'm currently running Achilles.. The cheapest tires I could put on my Ferrari.

 

QFT.

 

retro.phit
There is a range of solutions, and the path you chose is probably going to suit you just fine.

Mitch
Keep up the good work.
Not everyone cares about the finer points of chassis set-up, but some of us do.

I'm running 225/40R18 and 275/35R18 on 360 starfish.
Rake was been adjusted to suit this set-up many years ago.
More recently, the front got a smidge of additional negative camber. This made for improved turn-in and a bit more rotation approaching the apex. It also eliminated a tiny bit of fender rub, which was due to the ET31.5 front offset of the 360 wheels.

It took a while to nail it, but now the chassis set-up is optimized for my needs/wants/purposes.