348 Ideal 18” Tire Sizes

What you want (based on the last sentence) is to maintain the same rolling diameter and the same footprint.
Footprint is the first number in the tire spec.
Rolling diameter is: wheel+2×(footprint×aspectratio/2540)

Rolling diameter = 18" + 2×(215×45/2540) = 25.62

You can do the rest of the math.

If you change the footprint ratio (215/255) you will alter the understeer/oversteer relationship of the car.
{You can make up for small discrepancies with air pressure or suspension alignments--but that requires tools and work}

 

You have to understand the chassis. Ferrari 348 has the property that the rake of the car determines the understeer-oversteer relationship given a set of tires.
a) lowering an end of the car increases traction at that end
b) adding width to tires on an axle increases traction at that end
c) these cars are setup to slightly oversteer* when cornering under acceleration
d) when the car is "in the right ball park" you can fine tune with air pressure
e) all of the above only work when the car has been corner weighted

So, without knowing how the suspension is currently setup, what tires are on the car, and whether the current setup agrees with the drivers desires; nobody can give you advise--other than stick with factory settings/recommendations.

However, with (a,b,c) above, you can pick any reasonable set of tires (that fit under the fenders) and get the car to drive and handle well--and without changing the anti-roll bars.

I could put 255 fronts with 305 rears on my F355 and be "in balance" but this would require wheels with wider rims. This, in my opinion, would be "too much tire" and degrade from the driving experience.

Me, personally, I did add 10mm to the rear tires of my F355 (275/40ZR18) because there are so many more tires available in that size than in 265/40ZR18s, the fronts stay 225/40ZR18. This cause me to lower the read by 1mm since the 275 raised the rear by 2mm and lowering by only 1mm left 1mm of additional rear ride height; compensating for the wider rear tire.

 

Right, Mitch. As you said,we're concerned with the car's understeer-oversteer properties. I'd like to add this Racing Setup Guide to your post which includes other variables involved.

Barry

 

Just remember: you added 20mm to the rears, so you need to use (215/255)×275 = 231.8 as the rears.
225s will understeer.
235s will oversteer.
neither will be outside of the window where air pressure can be used to tune the relationship.

 

Personally I think the 355 size is great, its what I run. Personal experience. That said I have seen 295's on a 355 and it looked well amazing. Just too much tire for a 348 though. One th ig about hte lower profiel front is it lowers the front lip. Notice the 355 has a much shorter front bumper/lip, its not as low as the 348. The 348 is like a vacuum when lowered, when you brake hard the lip can get mighty close to the ground. Best of luck, post it up when decided!

 

totally understood it was my concern too. If the front too high it tends to list the car and get wonky at speed. You local roads can determine if lowering is an issue or not is another thing. In my area is not optimal. In AZ it makes 0 difference roads are great. Not a pot hole to be seen and as smooth as poured concrete.

 

The continental extreme contact are quite good as well.

There are some reviews out on tire rack as well.

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Great you found some options in those alternative sizes best of luck!

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I just completed finding Diablo tires in 18" (it is tough these days) so I am surprised by the difference in a similar era car:

245-35 Front
335-30 Rear

And don't get fond of Pirelli tires....there are some sizes Tire Rack has had on "back order" for 3-4 years. I got tired of waiting and ordered Continental extreme contact.

 

The Pirelli P Zero's are nice, but they wear down fairly quickly....at least did for me, and I didn't even drive it hard.