Aerodynamics are important but unless all the roads are flat like a railway (which, of course, is infeasible) they really only impact trucks driven at high speeds with light loads. Of course any 5 or 10% savings on fuel is very helpful, and most North American companies have used aerodynamic skirts and shapes to minimize resistance, but when you're hauling 80,000lbs the density of air in front of you is dwarfed by the rolling resistance of the load behind you.
Where I live, trucks are governed to 105km/h (approx 65 mph) and frequently companies cap them lower than that, so it's not like they are driving around at 75mph. No company could afford the fuel bill.
I'm not saying aerodynamics isn't important, but only that significant changes need to be made that smooth the transition between road and rail so that single truck/trailer combinations aren't being used for stupid trips across the continent that could easily be accomodated by rail and are only used for local deliveries.
However, making rail responsive enough to tie into the just-in-time manufacturing sector is more difficult than it should be.
Cars yes, but with diesel over $5.00+/gallon there are very few trucking companies that will allow their drivers to go over 65 mph. It's simply too expensive.
The ones you see driving around at 70+mph are usually owner-operators who don't have their trucks governed.
Anecdotally, as someone with a lot personal experience driving an RV down the road capped at 65mph, I get passed all the time by big trucks. I do see plenty that are likely governed at 65 though.
