Usually I avoid fiddling with suspension and LSD stuff, I only mess with my cars transmissions if I need more acceleration or top speed for the track.
Here are the tips I can give you for transmission tuning(I'm assuming you're running on fully tuned engines and maximum weight reductions);
-Slide the 1st and 2nd gear all the way to the left to minimize wheelspin on these gears;
-For the other gears, slide them to the right side progressively with each gear, watch the lines on the graphic at the side of the screen, make sure there's a tiny little bit of overlap with each gear.
-The car should be able to reach the higherst RPMs possible on the final gear it has on the track's longest straight line on its own without slipstreaming, however the revs shouldn't bounce at the Rev limiter as you lose speed with that, so you will need to fine tune it.
On some tracks you will reach higher top speeds than others as long as you remember to do the last tip, so I recommend you to have a setting saved for tracks with smaller straights, a setting to the tracks with longer straights and a setting for tracks in-between.