On Road Testing: Air Density Calc. Info.
 

I'll try for :
National Weather Service Surface Maps w/ ISOBARS and SATELLITE COMPOSITE Surface Analysis
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During interstate, long, road testing, data like this, along with online air density
calculators can help get a sense, and sort out how environmental factors could be influencing observed performance of aerodynamic modifications.
https://www.wpc.ncep.noaa.gov/sfc/satsfc.gif

Why not use a barometer or altimeter for air pressure, a temp gage and do real time calculations in some sort of density altitude app on your phone?
Use google maps or 5 sattelite gps for altitude corections
Isn't density altitude not the same as what you are creating?

So when and where is the best place for highway mpg in America then?

My guess is hwy 70, 80, 40, 10(?) about 100 miles east of the rockies to the Mississippi during the summer. High density altitude on a hot day reducing surface drag. A/c necessary might negate that effect.

No actual experiences east of the Mississippi but you want to be away from the glacier moraine. Cant advise any north south roads, haven't done much of those that are flat.

I get pretty good mpg on the main interstate outside of flagstaff Arizona. It’s 7000 feet elevation, super dry with no humidity, and a mostly flat interstate.

onboard barometer / altimeter
 

1) I've seen one laboratory-grade barometer in my life ( CAR and DRIVER ).
2) We need a 'raw' barometric pressure for online density altitude calculators.
3) An aneroid-type altimeter, calibrated at the beginning of a test section, would lose calibration if one traversed from one isobaric line to another, or if pressure changed during the session.
That would be the value of the NOAA isobaric contour maps. You'd know if the altimeter had drifted. And density can be derived directly from millipascals, inches, or millimeters of mercury reported.
4) GPS will give you an accurate elevation, but it won't inform about air density.
5) Contrary to popular belief, the gravitational constant used in the air density calculation varies with location on the planet. There's no onboard instrument to quantify that which is available to us.
6) A sling-psychrometer and a psychrometric chart would nail down dry-bulb, wet-bulb, relative humidity, etc. which would define the specific weight of local air, from which density could be derived. It's not something you can do on the fly though. Even with a co-pilot riding shotgun.

For our needs: Unless youre using lab grade everything else $$$$, a lab grade baro is kinda like using a shotgun to kill flies in your house. Aanndd it doesn't have to move. All the other components of such a density device are correctable from other data points once you have a baseline. I would think the typical map pressure sensor dislocated would be accurate enough, there's also a knob on the altimeter to reset the altitude as appropriate. We are looking for changes here, not micro inch density accuracy at some point in the trip.