Optimal acceleration is had at roughly 80% load. Optimal rpm differs based on engine design. Most cars seem to be most happy between 1500 and 3000 rpm based on BSFC (brake specific fuel consumption)
charts that I have seen. In a manual transmission car it is very easy to stay in this range.
If you drive an automatic, you are going to have a hard time keeping your rpm below 3000 and staying around 80% load. In this case you can use a technique called fake or false shifting where you accelerate at the 80% or highest load you can without having the engine downshift. When you hit the top rpm your aiming for, you let off the gas and let it shift to the next gear up. You then reapply the throttle to get 80% or as high a load as you can without making it downshift.