ultimate tyres size for handerling?

[judge]

few changes happerning with my race car.
going from the 8x15 and 9x15 split rims with full slicks to some road legal rubber.
got a few options but am having a hard time working out what is going to be best.
this is a pure race car but i want it road legal so i cna do testing and shake downs esayer.
the modern trend is to go for the biggest rim you can fit with low profile rubber as aposed to the old school way of a smaller wheel with a higher sidewall.
i need to keep the rolling radius up so it wont mess with the gearing to much so a tyre between 600mm and 640mm is needed
that means 205 60 15/225 45 17

looking at the smaller super rally cars there all running 17/18 rims and super stiff suspension. have aroud the same wieght 750kg but not as much power. i have 150bhp@ wheels.

but looking at the classic porsche rally cars there all on 15" rims with a higher sidewall, i know that the regs mean you cant have low profile tyre.

what i need to work out is in a pure performance term will a 17" tyre and wheel be quicker than 15" one?
car is used for hill climbs and sprints so i need a super sticky tyre and very fast direction changes which lead me to the 17" set up. but talking to a few folk who say that i need some sidewall to allow me to gain some mechanical grip, which means the 15" route.

16" is not a option as the tyres i want to run are ether 15 or 17.

very confusing subject but any ideas?

cheers rob

[FJCamper]

Above: We run 15's. And we win.

Hi Rob,

You didn't mention if you had a torsion tube or McPherson strut front suspension, or a swing axle or four-joint rear suspension.

However, I recommend 15" wheels.

The large-wheel, small profile street tire is mostly cosmetics, but has a real racing origin. The low profile tire has less sidewall flex and better overall road course handling, if on a suspension made for it.

With a McPherson front end and four-joint rear, you come closest to being able to use wide 17's. But only just close.

Stick with the 15's. You'll be better off.

[judge]

its on a 1303
front end is fully rose jointed and adjustable for camber ,castor and toe plus ride hight
rear is on 944 trailing arms and spring plates so ride hieght is adjustable as is camber and toe.
have adjustable shocks going on for this year as well.

only down side i can see with the 15" rubber is to get the smaller side wall i will be going to a 580mm radius which will lower my top speed to around 100mph@7000rpm

this is the car at the moment

[gerico]

FJ,

What brand tires do you run for road racing and for autocross? If my memory is correct you run 175x15 at all 4 corners...?