pacejka parameter sets define how tire forces are generated, they have nothing to do with performance, you can tune parameters manually but the parameters have no physical meaning. you can observe the resulting curves and/or try driving each time you make a change.
there are 3 tire models in fgear: simple, tire96 and mf61.
they have different complexities but if you do not have hundreds of vehicles in the scene you can not tell the performance difference. you can use simple tire for non player vehicles and for player vehicles use the one that feels better for you.