the manual gearbox can gain time if you know how to change gear in a particular car - however the auto always knows how to change 'pretty well'
i actually found (on gt3 when i got the wheel) i was a bit slower with the wheel - maybe 2-3% lapttimes. i guess this maybe true on gt4 as well, but its a lot more fun if you're used to it. the forcefeedback is very true to life, with the tugging on the wheel when you're in a slide.. i recon a large drift in a rwd car is much more controllable with a wheel, but then thats never the quickest way round a course
for a laugh, try getting a highly tuned up clio v6, stick front downforce to 25, rear to 5, turn off all driving aids and try to learn to take it round a course. takes some doing
