I think the key word is cumulative, in other words if people become "A little" more conservative about fuel consumption, the cumulative effect can be meaningful. Not everyone can use the same tactics. I can avoid maintaining my speed on hills, which is almost constantly for me, run 65 on 65, rather than 70, up and down the grades. On the other hand someone in Springfield or KC or STL have other possibilities.
I think the bigger fear has to be the side effects, such as, does anyone want the government to start dictating what constitutes too much profit? Most traveling benefits a large number of low income people, these aren't people that can cut back, but people that have to go public assistance. The effects can be enormous in some areas.