I sort of found this out accidentally when I was messing around while hauling ore back to dock. Lets say you have your hauler loaded with 3 expanders for a max of 10km3. Then you load all your crap onto it and max out your cargo space. Now undock and turn off all the cargo expanders. Each expander is -10% to speed. When you turn them off, it would not give you that penalty. However when you turn them off, you can still hold everything that you originally did when the expanders were on. Your cargo would show up like 10000/5500. Now you can do your 20 jumps a lot faster then say with a -30% speed penalty.