17" Compomotive tire pressures

I just use the pressures stipulated inside the door jam. I don't remember what they are however. Car grips extremely well around corners. 235's on the front, 275's on the rear. Bites like a piranha.

 

There is no magic formula, but I try to stay with the factory recommended load rating, which applies to original tires, and apply it to the current tires. Almost all articles on tire pressure will say that nothing is guaranteed, different tires have different characteristics, etc. My explanation below is the same - just opinion.

I have 17 Compomotives and larger tires on my 328. Different tires have different load ratings, and all tires are different, so the size and manufacturer are important to know. Usually the tires' website will give you load ratings at each corner for different pressures. Pressure AND volume affect load rating. Aim for the same load rating as from the factory to start, and adjust from there.

The 328 factory tires load ratings for the original tires were:
205/55VR16 (1250@33 psi)
225/50VR16 (1406@36 psi)

To achieve about the same load ratings on my current tires, I use this (coincidentally, almost the same pressures):
225/45R17 (1250@32psi)
255/45R17 (1428@36psi)
The tires are bigger, but so are the wheels, so the volume of air has not changed much.

Here is another example for my Acura. I have two sets of wheels and tires:
P245/45R18 is 32 PSI front and rear.
P245/40R19 is 34 PSI front and rear.
The larger tire by volume is the 18 (less wheel, same height and width), so more pressure is required on the 19 to achieve the same load rating.

On my truck, original (sticker in the door):
245/75LT16 (rec 1736@ 37psi)
My larger tires (and more weight in the truck since new):
305/70R16 (1898@35psi)
I have achieved a larger load rating with less pressure, because the volume of the new tire is much larger (wheels stayed the same).

 

There are two things you trying to do with tire pressure optimization. First is to maximize lateral grip of the front tires and second is to optimize rear grip for personal preference toward understeer/oversteer.

Given a fixed suspension set-up and tires, front grip is maximized by adjusting the pressure so that the tires don't roll-over on to their sidewall or let the inside tread lift as the car is cornering. Your tire temperature measurements across the tread width is a great way to determine this as is simply "chalking" the tire by smudging a colored strip across the lower sidewall and tread. The chalk should scrub off to the margin between tread and sidewall but no more and your temperatures should be hot and even across the tread width.

The rear tire pressure optimization is a personal tweak. If you have too much understeer, you have to reduce the grip of the rear tires by either increasing pressure a little to reduce the thread on the road or decrease a little to cause the inside tread to lift a little. Both methods are reducing the amount of rear tread on the road while cornering.

If you have too much oversteer, do the reverse of this all: maximize rear grip and optimize front.

Very often tire pressures optimized for cornering are higher than those cited in the door jam. The ones in the door jam are often just the lowest safe pressures that provide the softest ride. The Ford tire blow-out issue, which was related to specifying too low a pressure to compensate for lousy suspension, caused all shops to inflate tires much above the door jam pressures.

In no circumstances exceed the maximum tire pressure molded into the side of the tire.

 

Hmmm... I have Nick's coilovers too. My car dances around quite a bit if going over lots of little bumps/uneven road. Maybe I'll try to lower my pressure too.

 

I think your car dances around on the road because you have too much rubber in contact with the road.

Originally a 308 fitted a 205/70VR14 Michelin XWX. In the 80's because of the carcase and rim design of Michelin TRX the car was able to run a very advanced low profile tyre; 220/55VR390 Michelin TRX. Michelin TRX Tyres

However Instead of having a side wall height of 121 mm you have now got an inch less side wall height with a different rim profile. You will always have an uncomfortable more skittish ride. People criticise the Michelin TRX, but mostly because they resented paying for them, or maybe because they drove on old perished rubber.

More modern tyres are designed to compliment more modern cars. Hugely wide footprints are not suited to a 308 if they were Ferrari would have fitted them.

What is used on a perfectly smooth race track is all together different to the road.

Another point to think about is if a 308 was suited to having wider rear tyres to front, en Ferrari would have fitted them

 

I'm far from an expert on tires but I think it is noteworthy that what was available to Ferrari in 1975 vs. what is available today is night and day. Assuming that the TRX tires and their associated size are best for the car because that is what Ferrari put on them in 1975 is myopic. Maybe that is what was best in 1975, but the tires that are available today are far better.
Is my tire size & combination best? I have no idea but I had the TRX tires on before I switched and the car rides/handles MUCH better than before. I also like the look of the fat rubber a lot better than stock.
Good luck finding people who are willing to spend $2000/set on tires developed in the 1970's. I would guess those who are, do it for originality rather than performance.
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I run 17" Compomotives with extremely wide tires and don't have the handling issues you seem to have. You might try adjusting your shocks (the Forza setup has adjustments I believe) I'm running with a set of adjustable racing Koni's with a fairly stiff spring and have been able to tune the suspension to my liking running 30-32lbs on the street.

 

I have attached a picture supplied by Michelin which demonstrates the way the TRX tyre and rim combination is able to absorb more bumps for such a low profile tyre.
the tyre and its rim complement one another, which is why Michelin made them millimetric rims, because you cannot fit a normal tyre onto a TRX rim.
The rim underwent a fundamental transformation, the essential characteristic of which was a flatter, lower flange. This new design of the rim and tyre bead resulted in a gradual curvature of the casing, without the "5" shaped flexing inherent in traditional designs.
I am not an advocate of over tyreing a car. it is derogatory to the handling. you may get more ultimate grip which for the race track is great but more progressive handling is after all what a Ferrari is about.
The proof for me is when Ferrari changed the tyre for the 328 they kept a similar size rear tyre but made the front smaller. If bigger tryes would have been better they would have fitted them. and they would have done a lot of testing before they came up with the tyre size they did.
Michelin are still making these tyres and they are made with modern rubber compound but the carcase still works in the same clever way which will make the handling of a 308 excelent when they are fresh tyres 220/55VR390 Michelin TRX of course old rubber doesn't work so well (i.e. more than 10 years old)
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