well for simplicity, a 10% larger final drive will reduce the top speed of every gear by approximately 10%, thus bringing them closer together.
this improves acceleration between gears as well, as usually in close ratio gear boxes, when you upshift, you'll be higher up in the torque/power band.
when deciding on a final gear ratio, focus on what you will use the car for. higher final drive ratio will boost fuel consumption as your motor will be running at higher revs than normal. (ie. 100km/h in fifth might go from 3000rpm to 3300rpm). if you're tinkering with the gear ratios for track use, then decide which gear you use the most and how long you want it, and adjust the final drive ratio accordingly. ie. if you take corners at 100km/h normally, probably better to lengthen second gear to about 110km/h.