Which will get better MPG, the 123 HP turbocharged Ford Fiesta, or the 74 HP, smaller, more aerodynamic and much lighter Mitsubishi Mirage?
When Ford announced they would be offering a 1.0L 3-cylinder turbo engine in the Fiesta in the U.S. market at the Los Angeles auto show in November last year, they said at the time it would have the best fuel economy of any non-hybrid car in America.
Well, guess what Mitsubishi has been saying about the 1.2L 3-cylinder Mirage?
They can't both be right, and here's why the safe money would bet on the Mirage coming out on top:
1) On the highway ...
It's possible the much more powerful Fiesta 1.0 turbo (123 hp / 148 lb.-ft.) could get better highway economy than the less powerful Mirage (74 hp / 74 lb.-ft.). The Fiesta's added torque under boost could be leveraged to gear up the transmission ratios and reduce top gear cruising RPM -- and supposedly 90% of this engine's peak torque is available at just 1500 RPM. Whether Ford will actually do this or not is the big question. Automakers do not have a track record of providing us with "economy" top gears in recent years (in North America, anyway).
UPDATE, Oct 29th: Fresh reports put the US EPA ratings for the Fiesta 1.0t at 45 mpg (US) highway, and 32 mpg city. So the answer to the big question is that Ford must have geared it way up to be able to claim 45 mpg.
UPDATE #2: In response to complaints and additional testing, Ford has revised down its Fiesta 1.0T MPG EPA estimates from 32 city / 45 hwy (old) to 31 city / 43 hwy (new).
2) Aerodynamics ...
Aside from gearing, the other major wild card in highway economy is aerodynamics. The Mirage has a lower (better) drag coefficient and smaller frontal area than the Fiesta. Mitsu is reporting Cd figures ranging from 0.27 to 0.31 for the Mirage depending on drivetrain/trim (0.28 for North America). However this site says a Fiesta 1.0 has a Cd of 0.328.
UPDATE June 26: Ford has been tweaking the Fiesta's aerodynamics. Though it hasn't released drag numbers, the highway fuel economy of the 2014 Fiesta SFE model in the U.S. was boosted 1 MPG through aero tweaks. So it's likely the 1.0 car will beat 0.328 when it arrives.
3) In the city ...
In city driving, it's unlikely the Fiesta will get better fuel economy, because it's quite a lot heavier than the wee Mitsu. In Australia, the Mirage 5-speed tips the scales at only 865 kg / 1910 lbs, and in the U.S./Canada it's just 1973 lbs (it has gained a seventh airbag, further sound-proofing, and also a beefier rear bumper).
The U.S. spec 2013 Fiesta hatch 1.6L 5-speed weighs 2537 lbs, and the automatic porks out at 2575! Even if the 1.0L manual sheds a little of that weight, they're really in different classes altogether.
See also: 12 lightest cars in the US, 2014 (vs. Mitsubishi Mirage curb weight)
4) Dollars per gallon ...
Then there's the price question. Turbo kit ain't cheap, and Ford is positioning the Fiesta 1.0t as a mid-range model. US prices start at $17,240 in sedan form and $17,840 for the hatch (including delivery fee).
The additional $2,475 cost of the Fiesta 1.0 turbo over the base 1.6L car isn't recovered in fuel savings over 8 years, according to a Cars.com "best bang for your buck" study (the Mirage took the top spot).
On the other hand, the Mirage is a true cheap & cheerful econobox: the cheapest trim (~$14k including delivery) 5-speed may not get the top MPG rating the CVT earns, but there's a good chance it will still beat the Fiesta 1.0 at the pumps in mixed driving, and definitely by thousands of dollars at the dealership. So if your objective in saving fuel is saving money...
5) Smiles per gallon ...
The answer to this one depends on what makes you smile. Saving money? Or flicking around a small car that handles well?
Let's face it, the Mirage is not known for its sharp handling, while the Fiesta has a reputation as a bit of a driver's car. But guess what happens to the Ford's MPG when enthusiastic drivers keep dipping into the boost? Can you have your cake and eat it too?
The Mirage can be fun to drive in a nerdy way: it responds remarkably well even to basic eco-driving techniques (especially in the city) and can surpass its mileage ratings by a surprising amount.
Advantage: depends on what you're after
In the end, ain't competition grand? It strengthens the breed.