Adrian the final drive is the final gear in your gearbox. It changes the overall speed of the final output speed of a gear (the final drive). If the ratio is higher, like the EuroR's 4.75 to the CL9's 4.38 (I think that's what Aaron said it was) then it takes approximately another .4 revolutions of the engine to make 1 revolution of the driveshaft, thus the engine needs to spin more to get the wheels to move the same speed, giving you a lower top speed in each gear.

The reason you want a shorter final drive is so the car can throw itself through each gear quicker because the gear isn't as long. But usually the shorter final drive is used in the Type R's where they're tuned for high rpm and they don't want to fall out of peak power after a shift. The K24A makes it's max torque at 4500rpm, and after a redline shift from 1st into 2nd you fall basically (I do) right back to 4500rpm. The CL9 box is pretty well suited to the power curve of the engine and I don't think the EuroR final drive would give you much benefit at all.