If you are going, say, 300 miles (which happens to be the length of a trip I make occasionally) it would take 6.4 hours at 47 and 5.5 hours at 55- a .9 hour difference. We then need to figure out how much better the FE is at 47 vs 55 and then we can draw a conclusion as to whether creeping along more slowly really pays or not.
You drive 55? Really? I'll admit, a lower Cd is preferable... for those subsonic trips! LOL! Nobody has mentioned smooth wax jobs! Or, rolled up windows! Or, disc wheel covers (spokes are neat, but expensive in term of turbulence;ie, drag!)...spun aluminum discs from JC Whitney (the screw-on type...but GLUE 'em on with RTV!) are ideal! A hook-type device comes in real handy when you're beside the road with a flat tire(jerks the silicone seal loose!)... just hook into one(all?) of the screw holes! A good, uniform bead of RTV is super-strong! It will not come off by itself!
At approximately that speed, the contribution of aerodynamic drag is roughly half the sum of forces resisting a vehicle's motion (aero and rolling resistance).
Aero drag still exacts a fuel efficiency penalty at slower speeds, although proportionately less (in other words, it doesn't "suddenly" become a factor at 47 mph). Even at only 20 mph, it represents roughly 1/4 of the sum of aero+rolling resistance opposing vehicle motion.
Also, 47 mph is quite a high threshold (where aero forces start to exceed rolling resistance); That would represent a very aerodynamic vehicle - or a vehicle with very high rolling resistance. For most, I would say the 50/50 point is actually closer to 35-40 mph.
To clarify a "clarification" : P=1/2 rho v squared is a pitot tube formula , out of the history books (Andre' Pitot, building a bridge over a fast French river, invented this device... sometime in the 1700s). Since air is a fluid, it follows the same general rules of fluid behavior...and can be referenced for comparative purposes. I had installed a simple pitot tube / manometer test rig in a Gremlin I used to have ; deflection of the food-colored water occurred at 47ish MPH (Gremlin speedo). The pressure end of the tube was located in a temporary aluminum panel, replacing the right headlight. I was just making a point ; who has the time or facilities (or money!) to install a full bellypan?
Nobody has mentioned smooth wax jobs! Or, rolled up windows! Or, disc wheel covers
Disc wheel covers have been covered extensively on this site. Rolled up windows have also been discussed, but not extensively because it appears to be rather common sense.
I personally don't know how a smooth wax job is going to do anything other than make my car look nice. Until I see MPG numbers before and after a good wax job I'm not going to believe that it can contribute to mpg on a statistically significant level.