EDIT: No longer applicable since the 1.23 update.
I was initially very disappointed with GT5, however, after following Calan's method for choosing tyres I started to have fun with the game. Maybe we can find a similar work around for GTS...
Been testing with the Ferrari 458 Italia on Suzuka, & here are my first impressions & findings. Please note that I'm only testing untuned road cars, with the aim of trying to find a way of fixing some of the tyre issues without resorting to tuning. This is because I'd like to race them online in Tuning Prohibited lobbies at some point in the future.
Initially, I thought that the 2nd gear ratio was way too short on the 458, but when I fitted grippier tyres it went back to normal. The tyre model is so bad that I didn't even realise it was supposed to be simulating wheel spin, it just felt like a short gear ratio!
Even with hard race tyres all round, the back end still steps out in 3rd gear when cornering. The only solution I've found to this is to fit SM to the front & R1 to the rear on the 458, this balances out some of the flaws in the tyre model. I kept going up a tyre compound & retesting until the sliding rear in 3rd & 4th all but disappeared, but didn't go beyond that as the car still needs to retain it's character (which it does imho). I went up 1 compound on the front as well to get rid of some of the typical GT oversteer.
Traction control set to 1 solves all of the wheel spin in 1st gear for grid starts, & I also noticed that the traction control never kicks in at any point after this no matter how hard I push or slide the car on the SM/R1 set up. So, I'm leaving TC on 1. If that bothers you, use TC merely as Launch Control & then turn it off once the race has started.
I think the solution for now for RWD cars is to fit grippier tyres on the rear, & less grippier tyres on the front, completely ignoring any prejudice we may have from previous GT titles regarding the "Racing Soft Brigade". Test it by going up 1 compound at a time on the rear until the problem disappears, then go up one compound at a time on the front
only if there are any understeer problems. However, make sure that the front is at least 1 compound lower than the rear, otherwise the sliding issue in 3rd & 4th may return. You'll probably have to reverse this method for FWD cars. I haven't even tried 4WD yet...
EDIT: I also noticed that loading the front tyres by coming off the throttle a little, or even fully engine breaking through the esses had little to no effect on understeer (with the default S1/S1 tyres). Something is very wrong with the tyre model here too, as it doesn't seem to be simulating longitudinal loading all that well. More testing is needed...