19" wheels - difference between 35 and 40 profile

The road car tire nomenclature Doesn't make sense to me. It is a mix of metric (tire width) profile ratio (height), and inches in diameter of the wheels.

The profile of 35 for instance is 35% of the width.

For example the front tires of 360 is 225/40/18. This means the overall diameter is
40% of 225mm = 90mm.
The rim diameter is 18"x25.4mm= 457.2mm
Overall diameter of the tires are 90mm+457.2mm+90mm=637.2mm or 25.09

For F430 225/35/19
35% of 225=78.75mm
19" of rim diameter = 482.6mm
Overall diameter = 482.6mm+78.75mm+78.75mm=640.1mm or 25.2"

So it is almost impossible to go with the same profile tires when you increase the diameter of rim, unless you get skinnier width tire. Which I assume you don't want to.
It is important to maintain the overall diameter close to factory due to clearance issue. It is important to maintain the ratio of the front and rear tires overall diameter for proper function of traction control.

The racing slick nomenclature makes more sense as they don't use profile ratio but overall diameter. Still mid of metric and inches, though. For instance 240/640/18. The 640 in the middle is the overall diameter.

 

here is a helpful formula I use to discover overall diameter. It can be helpful to access the change in what you have to what you are going to. In my experience the slight change in sidewall from 35 to 40 is not very much... don't know if you will "feel" a difference.

I have 19's on my '03 Modena


235 x 0.35 x 2 divided by 25.4 plus the rim size 19"
235 - 35 - 19" in the formula =25.48" overall diameter of the tire.

With the front tires at 235 x35 x19" on my car you have to be careful how low you go up front with the suspension. I "pulled" the inner fender lip a few times already and I will be rolling the lip here soon.

I feel for you. I feel the road pretty good without really bad conditions and with the Sport setting off.
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Having a shop for over 25 years I have played this game of maximizing tire size to clear on the many cars we have built. And it is tough to figure without all the variables of offset and ride height considered and nailed down. I have even found certain manufactures of the same size tires have a little wider tread width than others-- and most times this is where the "snags" will happen-- not at the side wall where the industry figures width.

Going from a 235 to a 225 is only going to be 5mm less (only about a ¼") to the outer fender opening so not sure how you juggle the ride height on that but it really is not that much-- and that is at the widest point of the sidewall which is not usually where the tire "snags" the inner fender. Will a ¼" at the side wall help if you "grow the tire height" bringing the sidewall closer to the inner fender lip. Tough one to call.

FWIW: I have the Michelin-Pilot-Super-Sports on my car now. Of course the ride height and offsets you run have a lot to do with clearances. The photo of the rear of my car is with the Forged 360 (brand) wheels with the application for the 360 and 430s. They are very wide wheels (12.0" I believe) and I am running 325x30 -19" with no issues of clearance. I only bring this up to show how offsets and ride height can really effect things when up-sizing tires.

FWIW: up front the wheels have about a 12mm (½") spacer on the back to bring the wheel /tire set up out some to give a more "full" look-- and as I said with a 235x35's up front I bite the inner fender lip on occasion. I may look at removing the spacer from the back of the hub and I'm sure then I would have no issues. So here we go again in playing with offset and not changing the tire size. All factors have to be considered.

Sorry I can't really compare the "ride" of this set up to the factory 18" wheels since I changed the wheels when I got the car. So I don't know if any of this helps.

Cheers, ken

 

Its not due to the tyre width, its the offset of your front wheels and the width of them. They probably fill the fenders which is why you are catching your fender lips.

 

i put OEM 19s on my 360. No problem. Did have to role the fender lips to prevent it from catching on the tires

 

I put the 19" challenge wheels on my 360 that had 18" starfish before, both with pirelli tires. I felt that the 19" had much better ride, but I suspect it is possible that the 18" weren't properly balanced. To be fair, in southern california we have pretty good roads compared to the rest of the world

 

Of course with lower tire profile you will be planted to the road better. Less wobbly. I run 19" at my 360 and 35 series tires. Works great. I predict there might be rubbing at the front with 40-series. Especially if you have lowered to CS ride height like I have...

 

I think the car had been sitting and developed flat spots on my old 18", that's why the 19" felt better