Rear Tire Size 245 vs 255/40/17

i would say a couple things.
1) You won't notice any difference in handling 245 to 255 rears. You could probably measure it on a skid pad or drag strip, but I can't believe you'd feel it. you will fell 225 v 205 fronts, the wider you go the heavier the steering and its about proportional so 10% more effort. If you add a quick ratio rack that is 2.5 turn v 3./2 turn that is 20% more effort.....which is why there are power steering kits.

2) you do not want the OD of the front tires larger than the rear, that looks awful. Performance wise I don't think it matters....but the car looks 100hp slower.

3) changing wheel/tire size means you are re-engineering the car's suspension. This leads to all kinds of not simple topics

Gong wider in the rear generally means you're adding rear grip so the car will understeer more which is safe but can be annoying. The early 328 is very similar to the 308s which were designed in the early 70s and the suspension mostly followed the 206 from the mid 60s so its an old design. The street cars had matching front/rear wheels tire which is great for spare tires fitting and rotating tires and such but making the a car with about a 42/58 weight split on mine but maybe stock is more like 44/56? On a skid pad the tire widths should be about proportionally to the weight split so at 44/56 a front 225 matches with a rear 285. I ran 225/285 and now run 225/295 and its fine. The GTO was 225/265 and race cars were running 8in front wheel with I think 12" rear wheels? That's a lot of variation and they all work when the suspension is retuned to match which is pretty key if you want performance to match the looks.

The factory suspension tuning is REALLY strange to make matching f/r tires work and get the ride the way the marketing team wanted it. The suspension get a lot stranger when you lower the car , body roll goes up significantly (because the roll centers (which help control body roll drop and no longer help much) so you need stiffer springs and/or swaybars to compensate....I not saying you need to bump up springs and swaybars when you low the car, just that there will be more body roll if you don't.

This is a lot of info.....but customizing=re-engineering so......

 

oh. let me add, you were talking about hp in the other thread....adding 50% more hp also changes how much rear grip you want/need. The race cars had both more hp and less weight (so much higher power to weight ratio) which helps explain the 8/12 wheel sizing. An honest 400+hp 328/355 hybrid engine will bring a significant change in how loose the rear feels coming out of corners compared to an optimistic 260hp 328 engine.

 

For tire measurement you really want to use the manufacturer's published specs. A calculator will ballpark you but they all cheat a bit, particularly with performance tires. Here in the US i use tirerack.com, they have a lot of data that's easy to pull up.


Here is a power steering thread to get you started
https://www.ferrarichat.com/forum/threads/electric-power-steering-for-your-308.616456/

I'm getting ready to offer parts but more to race/hotrod market and was planning kits so probably easier to buy the ezpowersteering kit.

 

Careful believing shock company claims....they are usually sort of true

For sure you can adjust ride height. For sure they can make it easy. But also for 100% sure the lower you set the ride height the stiffer the springs need to be to keep the car off the snubbers. There are lots of snubbed options so you can use the snobbery as helper/progressive springs....but 9nly after you understand that you are riding on the snubbers and need to tune then.

Then if you are riding on the snubbers and effectively have progressive springs you need to understand that you also have progressively less damping factor. Normally progressively suspension is done with linkages so the spring to damping factor is constant. So tuning snubbers works but there are compromises.

Last, trying to use the shocks to control body roll is a fools errand. Don't believe anyone who tells you they can do that. Shocks can delay body changes and that can be helpful, but there job is to keep the tires on the ground so generally it's best to just let them do their job. Springs hold the car up. Swaybars, and roll centers tune body roll and allow fine tuning f/r tire loading to tune under/overseer.

That is all perfect world actually racing stuff. For sure you can tune your car to drive decent on the street and lower it at shows or smooth streets to look meaner. Also for sure you can tune you car for the track and raise it to save your balance on the street. But actually tuned for both.......

 

These are what’s on my 17s now
Front: 17x8 front 225/45/17
Rear: 17x9 rear 255/40/17