Fuel efficiency and fuel economy are different things.
It is not so, that the passive hybrid implementation by GM for a large vehicle has reduced fuel consumption by half.
Regardless...if a vehicle's fuel consumption is halved, you use 1/2 less per week. Right?
Egregious advertising claims always will exist - such as Toyota's for their Prius model.
Fuel economy factors remains as always - weight moved, internal friction, rolling friction of tires, tire chemical composition and construction, management of wind resistance and the allied attendant frontal area including tire width and vehicle speed.
Gentle acceleration to speed, lifting off the throttle when approaching incipient stops, are big players in fuel usage as always.
Despite some who wish otherwise, the "sweet spot" remains 30 to 35 mph.
A national reduction in limit to, say, 40 mph would be profound. Certainly, the 1st effect would be the delay in truck goods delivery, and a sea change for industries that utilize the just-in-time manufacturing methodology.
UPS delivery, the OTR truckers would take a quick hit.
In the long term, with the slowing of transportation - business and personal - "off-Interstate" businesses would renew. The motels, the small, non-chain restaurants will reappear. The cities regain importance at the end of fast runs to the suburbs. City property values are restored, the criminal element eventually gets out of town. City tax revenues rise - they are rebuilt to the stature or yore.
And...we now get to buy and drive stylish boats. Chrome bumpers. Crisp fender lines. Fins. Convertibles. Glitzy interiors.
The end of the jellybean cars...