>>> Short answer? It depends...
Despite the CVT's better rating, it's not necessarily the most efficient.
Long answer? Read on...
Until we can convince Mitsu that they need to offer us a MUCH taller, "ECO 5th gear" option in the manual transmission (thereby guaranteeing the car's cult status among economy car enthusiasts)...
There is the question of which transmission will deliver the best real-world results when fuel economy is the driver's goal.
We know the 1.2L with CVT is rated better in both city and highway driving than the 5-speed manual. The US EPA (currently the most realistic rating system) says:
Mirage 1.2L fuel economy
City / Highway / Combined CVT: 37 / 44 / 40 mpg (US) 5-speed manual: 34 / 42 / 37 mpg (US)
Which one you should choose is not a given, despite the higher ratings of the CVT. The manual can outperform it in the hands of a skilled, motivated driver in the right environment.
So, to decide, find yourself in this list of scenarios ...
>>> If you're a "typical" driver (turn key, turn on tunes, go go go)
The CVT is the one to get for best fuel economy, regardless of where you do most of your driving.
Just try to keep engine RPM as low as you reasonably can (by not flooring the go pedal), avoid rushing into situations where you must brake, and you'll get decent results. (Thread: Rally driver gives fuel economy tips for driving the CVT Mirage (Video) .. )
>>> If the majority of your driving is on the highway
Under most cruising conditions on the open road/highway, the CVT's taller gearing keeps the engine spinning at significanly lower RPM than the manual, where it burns less fuel. [ See thread: Gear ratios: 2014 Mirage transmissions, 5-spd manual & CVT (speed vs. RPM chart) ]
Yes, there is a chance that the 5-speed could beat the CVT at highway speeds using a specific, advanced driving technique called "pulse & glide" or "burn & coast". But the technique is tedious when used repetitively, it's inappropriate to do in traffic, it's potentially dangerous for inexperienced drivers, it's illegal in some jurisdictions, and it's arguably harder on the car (mechanically) than simply cruising along at a steady speed.
So the CVT is the highway winner. (For now.... UNTIL we can convince Mitsu to offer us a proper ECO 5th gear!)
>>> If the majority of your driving is sub/urban
I would bet the manual can outperform the CVT in the hands of a competent and motivated eco-driver in sub/urban driving. (Where speeds vary, and the highest constant speed is no higher than about ~75-80 km/h / ~46-50 mph).
UPDATE, Oct. 28 -- I've had the chance to drive both transmissions and compare mileage on the same route. As suspected, the 5-speed beat the CVT. See: Gas mileage/MPG test: 2014 Mirage CVT vs. 5-speed (sub/urban Ottawa route)
2014 Mitsubishi Mirage 1.2 L Observed fuel economy 5-speed manual 48 mpg US
(4.9 L/100 km = 20.4 km/L = 58 mpg UK)
CVT automatic * 42 mpg (US)
(5.6 L/100 km = 17.9 km/L = 50 mpg UK)
See comparison test for more information.
The manual gives the driver much more control over RPM vs. engine load (a loaded, low RPM engine generally produces power most efficiently).
Also, the ability to use neutral for coasting up to transitions (stops & turns) is easier done with a manual. Sure, the CVT can be shifted to N just as easily, but going back into gear smoothly while the car is still in motion is something the manual was specifically designed for. The CVT, possibly not, so frequently shifting in and out of gear may not be good for it (or smooth).
Using advanced techniques will futher boost city economy: eg. shutting off the engine while stopped/slowing is a big one. Only Europe & Japan get the factory "auto stop and go" feature for now. Without it, we have to do this manually. And restarting the engine and moving away smartly (ie. not holding up other drivers) is much more easily done with the manual because the engine can be re-started while the proper gear is already selected, then ease out the clutch and go. (Where the CVT must be started in N or P, then shifted to the proper gear to wait for it to engage before accelerating.)
So, IF you're motivated and skilled (or willing to learn / develop your eco-driving skills), I'd pick the manual for sub/urban duty.
>>> The CVT's payback period / "return on investment" ...
The CVT in the costs $1000 extra in the U.S. ($1200 in Canada currently). Going strictly by the U.S. mark-up and EPA ratings, and with fuel cost at $USD 3.61 per US gallon ...
If you drive exclusively on the highway (15k miles/yr.)...
CVT @ 44 mpg 5-speed Manual @ 42 mpg Annual fuel cost $1231 $1289 Annual difference -$58 +$58
Going strictly by the rating, it would take about 17 years for the CVT (at $1000 extra) to pay for itself in highway driving.
In mixed driving 45% city/55% highway (15k miles/yr. )...
CVT @ 40 mpg 5-speed Manual @ 37 mpg Annual fuel cost $1354 $1464 Annual difference -$110 +$110
And in mixed use, it would take about 9 years for the CVT (at $1000 extra) to pay for itself in combined driving.
In 100% urban driving (15k miles/yr. )...
CVT @ 37 mpg 5-speed Manual @ 34 mpg Annual fuel cost $1464 $1593 Annual difference -$129 +$129
And in mixed use, it would take about 7.8 years for the CVT (at $1000 extra) to pay for itself in combined driving.
Of course, if you drive more, or if the price of fuel increases (a pretty good bet), the payback period will be shorter. Play with the numbers here: