[BLSTIC]

Below 55mph aero is still a big thing if you're trying to maintain speed. I'd go so far as to say as at 35mph around half your energy goes to drag. In the 90's they had the top of the car more-or-less ok for drag, but the underside is typically a big mess (out of sight out of mind). Minor work on the front and rear (undertrays) can make a huge difference to the underside of the car. I'd say you could drop 35mph cruise consumption by 3-5% with a few hours work.

I think that 55mph aero myth probably came about because if you're not doing 55mph you're in stop/start traffic of some kind, where the (relatively) constant braking wastes most of your energy. Drag accounts for little of this kind of driving, but still has some influence (look at the detail on EV's designed for the city, where every bit of energy counts towards range. The opportunity for 10 miles of extra range outweighs the $50 per car extra it costs).

So yeah, if you're in a city, aero doesn't matter as much, but in terms of effort per economy gain, chasing the low hanging aero fruit is still worth doing.