[86TOYO2k17]

To over simplify it. The % difference between FD ratios will be the same percentage increase in acceleration but only to the top of each gear. Then once it shifts it loses time/power shifting/dropping in power band. It will also be the same percentage difference in the top speed of each gear and the increased rpm at any given gear. (With 10% higher ratio 2500 rpm at 60mph becomes 2750 at 60)

For example say a car with a FD of 4.0 and hits rev limit in 2nd at 70mph in 7.5sec (63mph in 7sec) and 3rd at 100mph in 14.5sec (90mph in 13.5sec)
The same car with a 4.4 FD will rev limit in 2nd at 63mph and 3rd at 90mph (10% less) however it should theoretically get to 63mph 10% faster then the 4.0FD would get to 63mph. So 63mph in 6.3sec and 90mph in 12.15seconds) but the time spent shifting and being lower in powerband will make the 4.4 slower to 70 and slower to 100.
The exact amounts vary by weather conditions, wind resistance, tire grip and other variables.
Basically decide what you are using your car for and then choose FD ratio based on that.