Need help in deciding tire size

Funny timing... I just pulled my old RE050As off my stock wheels this week. They are getting old (>3 years) and near the wear bars. In the meantime, I put on my other set of wheels with tires that I had bought as track tires when I had a 512TR. The tires are Pirelli P-zero Corsas (you know, the R-compounds).

Fronts: 245-35 18
Rears: 315-30 18

There is no rubbing or any fitment issue. The difference these tires make is UNBELIEVABLE. It's like a completely different car. The grip is unreal. It just solidifies my belief that 90% of the increased performance and feel of cars like the CS and Scud and GT3 are the ridiculously sticky tires.

That being said, old tires certainly lose their performance, and the new generation of high performance tires is blending the division between R-compounds and good summer tires. So you are probably going to be suitably pleased, regardless.

I will probably get 295s in the back, but use stock size in the front. It's a common up-size that works well.
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SKUSA

Check out Bridgestone Potenza RE 11 as alternative to PS2 and Supersports. On the Tire Rack ratings, it is listed as the best tire in the Ultra high performance summer tires. pS2 's are ranked near the top of the pack in the lower category-super high performance summer group. The RE-11's came recommended from a good friend of mine, auto cross champion, who has them on his 600+ HP 65 mustang with full Roush-Yates motor. He claims that changing from slicks to normal high performance street tires, causes anywhere from a 5-10 sec increase on his times on certain courses, whle changing from slicks to the RE 11's he only has a 2-3 sec increase in time. On tire rack rankings the dry grip, wet grip, and tread wear ratings are all at the top if it's class. The ranking where it is mid pack is road noise, but I doubt you care about this that much if you are runny a Capristo Stage 3 on an F355! The killer point of these tires is that they are also much cheaper than the Michelin ps2, ps3, and supersports. I have placed the RE 11's on my AMG and they are a great improvement to the super expensive stock Pirelli P zero that were on previously. I have275/19/30 on the rears. When the PS 2's wear out on my Ferrari I will place the RE-11's on all 4 corners if I can find the appropriate size. You should have no problem fitting the appropriate size since you currently have Bridgestone's on your rims. After Chrigel put the RE 11's on his f430, he swears they are the best tire that he has ever had. Reading many Internet reviews of people on the RE 11's, the common theme is repeated that " these are the best tires that you can buy if you are driving your car on the street and using it also frequently on the track, the best regular tire short of R-compounds!"

 

Tires is a very subjective topic - if you want ultimate grip then you'll have certain options, if you want to add a bit of comfort or quiet into the mix then it'll be other tires - then there are others still who care more for looks and would choose a optimal tire in favour of a tire that looks better or is wider.

For my wheel setup (18/19) I am looking at going to the Advan AD08.

o directly answer the OP's question about size, you should keep the OD of the tire as close to stock as possible, the 245/35/18 is close but if you are going 19" then there are no sizes that are close in a 245...

Maybe choose a stickier tire instead of wider -

 

IMO the way to do it is put the widest tire that will first be supported by the wheel meaning don't shoohorn a 315 on a 10" wheel. Second make sure you don't rub. For example I was able to run 245's on 355C wheels on a race lowered 348 racecar but my springs were really stiff and the rubber would just barely make contact on the inner fender lip under extreme motions. The rears are no problem. Then once you got the most rubber start at stock alignment or mild track alignment. Since you don't have adjustable sways, baskets full of springs, and 2x/3x adjustable shocks tune the over/understeer with rear chassis rake after you got the front set to stock height or the lowest height you like until you scrap all the speedbumps. You will probably not get to the point where you need to "bumpsteer" the car since this is mostly a streetcar and you'll probably not slam the car...right?

 

This is mostly true. But once you got the sticky tires you gotta have the horsepower, hence corner speed, to scare yourself all over again.

 

The car is loose due to the rear tires having such a small rolling diameter.

The 225/40ZR18 have a rolling diameter of 25.08
The 265/40ZR18 have a rolling diameter of 26.34
The 235/35ZR19 have a rolling diameter of 25.47
The 285/30ZR19 have a rolling diameter of 25.73

You raised the front and lowered the rear, screwing up the chassis rake.

A 255/30ZR19 has a rolling diameter of 25.02
A 305/30ZR19 has a rolling diameter of 26.20
The ratio of 265 to 225 is 1.1777
The ratio of 305 to 255 is 1.1908
This is the best fitment if you can get the tires on the wheels.

It will also get rid of the rake issue noted above.

 

The 245/30ZR19 has a rolling diameter of 24.78
The 295/30ZR19 has a rolling diameter of 25.96
The ratio of 295 to 245 is 1.20

This is an acceptable tire choice, but not as optimal as the previous 255 and 305. It will lower the car by 1/4 inch and a little bit more in the back than in the front. Either one should be close enough to tune with tire pressures.

If not you can lower the rear for more understeer or raise the rear for more oversteer. Something you can't do (as easily) when the rear tire is too low as in your current tire fitments.

 

Hey, that's what my ex-wife used to complain about, too.

 

The 245/30ZR19 has a rolling diameter of 24.78
The 245/35ZR19 has a rolling diameter of 25.75

Its not personal, its math. And its the difference between a 30% aspect ratio and a 35% aspect rato.

 

The 225/35ZR19 has a rolling diameter of 25.2
The 265/35ZR19 has a rolling diameter fo 26.3
The ratio of 265 to 225 is 1.178

This would be the ideal set (available in SS; I checked) but you should lower the front by 1 trun on the spring perch to make up for the extra height up there.

Slightly wider and still fitting on your rims and available in SSs:

The 255/30ZR19 has a rolling diameter of 25.20
The 305/30ZR19 has a rolling diameter of 26.30
The ratio of 305 to 255 is 1.178

Here, you should raise the rear by 1/2 turn.

The 245/295 would require the car to be raised 1/3rd of an inch to get back to the absurdly small 4.2" of factory ground clearance.

As to the relationship between wheel width and tire width. As long as you wheel is within the range listed at Trie Rack, the grunt at the tire mounting station can get them to fit. {And in a more essoteric sense:: the wider than wheel tires works well with the suspension geometry of the F355. That same calculation would not hold on a car with strut suspension. This is simply due to unequal length unparallel A-arms versus McPherson struts.}

 

I have the Super Sports on my Challenge rims. Once you get about 500mi on them the grip is unreal. I had Pirelli Corsas and I think these tires grip better and the ride is much better.
I had them at Road America with just a few miles on them and they were very slippery, but now I love them. You won't be dissapointed with these tires.

 

No if the rear is lowered the car sticks better. OP talks about loose in turns.

 

Don't match the tires to the oem suspension set-up of the stock tires. Buy the tire that fits the wheels best and readjust the rake and rideheight to oem as the starting place and tune from there.

"Wider that wheel tires" do not work if the rubber is not properly supported by the wheel, your laptimes will be slower.

Don't adjust grip by tire pressure. Since the OP tracks, find the optimum by testing the relationships between setting tire pressure by pyrometer readings, wear, chassis rake, and alignment settings.

 

With a given set of wheels and tires, lowering the rear by use of the spring perches, increases rear traction.

However, when the rear is lowered by a tire of insufficient diameter and the spring perches remain the same, the short rear tire generally suffers from oversteer.

 

Not really. Lowering is lowering regardless of how you get there. In your example by decreased aspect ratio of tire, the change of car balance (oversteer/understeer) is orders of magnitude more sensitive to chassis rake vs. small increase in stiffness (tire spring rate)from aspect ratio.

 

Ive had 295 35 18 rear and 245 40 18 fronts for a while and they handle great.

 

No, when you lower the rear of the car with spring perches, you end up moving the rear roll center more (1.6X) than if you lower the rear of the car by using short diameter tires. Its a geometry change rather than a translational change.

The change in the (rear) roll center, then, changes the front-to-rear roll stiffness which shows up as changes to US/OS balance.

 

Yes...we are saying the same thing...me as a racer and you as an engineer.