As far as tuning guides go, give these links link link a try. They will show you most of the tuning-related information you're looking for, but to summarize the GT5 tuning process, here are a few quick tips:
For any of the tuning upgrades that you can buy, more is almost always better. In other words, the more money you spend, the better your car will become. The problem is knowing where to put your money first, so that you don't blow all your credits adding 200 horse power to a car, when all your car needed was a slightly better set of tires.
I always start the process by looking at what sorts of cars that AI competition will be brining to a race, and calculating the power to weight ratio of those cars and comparing it to the best cars I have. The power to weight ratio is shown for every one of your cars (when you are in your garage). If your cars all have much worse PWRs than the AI, there is little you'll be able to do to win a race. BUT you might have a race car with race tires in your inventory that can win against the computer, even with a lower PWR. At that point, you'll just have to try it out to see where your car stands against the computer. It will probably be a close race. The tighter and less-straight the track, the more importantly tires, weight, and suspension tuning come into play. The longer, straighter tracks (Indy, Daytona, and LaSarthe) favor power and transmission tuning.
As far as handling is concerned, the weight of the car will provide a very rough indication how the car will perform. A big, heavy 2500 kg pickup truck will necessarily be less nimble than a 700 kg Lotus Elise, and will be slower around a tight track, even if it has a high PWR. If your car has a high PWR and a lower weight than the competition, then you stand a fairly decent chance of winning... roughly speaking.
As important as PWR and weight, there are races in the game (Nascar and 'Formula Gran Turismo', for instance) where you cannot tune the engine or curb weight of the car or (** see below), and where all the cars make similar horsepower and are the same weight. In these cases, you will have to do some research PRIOR to starting the competition regarding what sort of spring rates, chassis height, down force, transmission top speed, tire compounds, and driver aides (ABS, ESC, traction control, etc) to use. An incorrect car setup in these races can make the entire competition impossible to beat. These are tough races because you cannot simply buy your way to competitiveness, but that also makes these competitions fun to race.
** - oil changes, engine rebuilds, and chassis rebuilds can still be used to increase engine performance, even if you cannot add a turbo or soup-up the engine.
Here is alink for the NASCAR setup.
Here is a setup for the Formula Gran Turismo race (in the extreme series):
The transmission settings are different for every track, but here are a few settings that might help get you started:
- F 70
- R 90
- Top speed 242
- 5/5 (10/10 for bumpy)
- Spring rate 17.4/17.5
- Dampers E 8/8
- Dampers C 6/6
- Roll bars 7/7
- Camber angle F 2.0 R 1.0
- Toe angle F 0.00 R -0.20
- Brake balance 4/1
Most of the time, you don't need to get anywhere near as much detail for your car setup, compared to the F1 races. Early on in the game, you can more easily adjust your PWR and weight of your car to compensate for any suspension setup deficiencies.
The first power-adding mods that you'll want to try (which are also the cheapest) are:
- Engine racing chip
- Racing air filter
- Racing muffler
- First stage of engine upgrade
- Better tires. It doesn't add power, but it will certainly help.
- Sports suspension
If that's not enough, then you may also want buy the 1st stage of weight reduction, since that helps better both the PWR and curb weight of your car at the same time.
I hope this helps!