Firstly, you can set dead-zones if you feel it starts too soon. Secondly, and what seems to be what you're looking for, you can alter the "acceleration linearity" and "brake linearity". I'm guessing the slider further to the left will make it more sensitive in the beginning and less in the end, and the other way around all the way to the right. It can be the opposite, too - I don't really remember.
Also, are you sure that your wheel isn't reporting the gas to be 100% when it's one third in? It might be part of the problem.