What George said…
;-]
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The cheap thing to do is remove your roof rack if you have one, cross bars especially. They kill aerodynamics.
Small aerodynamic cars also have small engines which can generate less power and still be in their thermodynamic sweet spot so the difference between optimum mpg speeds between small aerodynamic cars and big unaerodynamic cars is not that much.
If your engine’s thermodynamic efficiency was constant at all power settings, then the slower you drive, the better your fuel economy would become. But engines are most efficient around 50 to 75 % full power and the best fuel mileage speed is a tradeoff between going fast enough to make the engine efficient and slow enough to not need a lot of energy to move the car.
Electric motors have a much flatter efficiency vs power curve than gasoline engines do and as a result, electric cars really reward you for using a low cruise speed. A Tesla model S can go nearly 450 miles on a charge if you are willing to cruise at 20 mph. Go slower and the range drops off, likely because the speed independent
fixed power consumption, lights, radio, ventilation, etc. becomes a significant percentage of the power draw from the batteries.
Source of graph: https://www.tesla.com/blog/model-s-efficiency-and-range?_ga=2.242201095.481001083.1494986426-1771250445.1494986426
Actually the back end of your CX7 is causing a large portion of the drag. Making the back a long pointy shape would help more. But that would wreck the utility of the vehicle and make it hard to back up… 
Yup, Like that Metro!
When gas was $4 a gallon, 3 cylinder Geo Metros were trading at a serious premium price on the used car market. They got better than 50 MPG, easy.
My Crown Vic shifts into top gear at 40 MPH The engine is ticking over at 1200RPM’s That’s the sweet spot…But it’s almost impossible to hold it there…ANY hill or acceleration demand, even a slight one, will result in a downshift
Yet, I get some of my best gas mileage on hilly roads. You are alternately climbing a hill that puts the engine in its thermodynamic efficiency sweet spot and coasting with the engine burning almost no fuel.
That’s why gas mileage records are usually set using a technique called “pulse and glide”, the engine being used intermittently at a high throttle opening to put the engine in it’s sweet spot and then coasting with the engine off to allow an average speed that requires very little power to maintain.
If you have a hilly road, instead of accelerate and glide, you climb and glide allowing for a more steady slow speed and even better mpg.
I agree with Kolby12309, but would add this to his response: the highest gear _for that speed._. Back in the day your options were 2 or 3 gears for an automatic, to 3 or 4 gears for a manual. On new cars your options are from 6 to 10 gears for an automatic to 5 to 7 gears for a manual.
My Jeep has a 9 speed automatic and one of screens on my dash display gives real time fuel economy information. On level ground the best mileage seems to occur at about 40-50 mph in 5th or 6th gear. This falls off slightly at highway speeds due to aerodynamics, even though I get to 8th gear (it will not go into 9th below about 80 mph).
That’s all true if all other variables are the same, but they aren’t.
To go 40-60, I need to travel the second class roads, the ones with traffic, stoplights, hills and curves.
On the highway I can flow with traffic at 70mph and use less gas for the trip than I would on the second class roads.
So, to get from point A to point B using less gas, it’s better for me in my area to get on the highway and set the cruise control at 70. For the record, 40 to 60 on the highway would not be safe. I’d be a rolling roadblock.
I love physics, but you must include all the variables to get an accurate answer.
Gas was $.55 cents per gallon but PUC/Agricultural tax exempt (no state tax added) diesel was $.44 per gallon in 1977. With state tax added it was $.48 The VW diesel Rabbit with 50+ mpg on less expensive fuel were selling.
Gas was $.55 cents per gallon but non-PUC tax exempt (federal and state tax added) diesel was $.48 per gallon in 1977.
I guess we can no longer delete an unfinished post.
Im used to cars with a max of 6 speeds in a vehicle, and the car i drive has 5 gears so around 50mph it is in top gear.
Don’t forget the Kamm back. A great example of an efficient car that uses this design is the old CRX. Mine’s sweet spot was somewhere around 65mph, and that was unfortunate because back then the speed limit was 55. 
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My 2002 Sienna got 24 mpg at a steady 70. If I slowed down to 65, it went up to 26 mpg. And, at a steady 60, it was nearly 28 mpg. This was very consistent over 200,000 miles.
Twice, before I found someone who could actually balance tires in Mexico, I had to drive to the border at around 50 mph. That car got nearly 33 mpg. the problem was at that high mileage, resolution for only 850 miles wasn’t very good. There are only two times to add gas in that short distance. So, I really can’t swear it was over 30 mpg. But, it did measure at 33.
Mexicans are used to slow traffic, even on high speed highways. Also, they are far less a**l then most US drivers, unless they are drunk or on drugs. So, driving 50 is not a real problem if you stay in the right lane.
It’s a useless discussion. Regardless of what the “sweet spot” is, we never maintain it for long due to traffic and other conditions.
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Clearly you have never driven across Texas or South Dakota. 
You can go for hours without seeing another car if you’re not on an interstate.
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