The Mirage / Space Star aerodynamic design is better than average for a small 4-door hatchback body. Mitsubishi lists drag coefficients (Cd) ranging from 0.27 to 0.31, depending on drivetrain and options, with the Japanese and Euro market trims having the lowest Cd:
That Cd contributes to the Mirage being the most fuel-efficient gasoline (non-hybrid) registered car in Japan.Reduced aerodynamic resistance (an in-house-measured CD value of 0.27 (on G and M trim levels)) - Source: (PDF) Social and Environmental Report 2012 - MITSUBISHI MOTORS CORPORATION
Mitsu U.K. says:
Notable aero design features (varies by model/options):The most advanced research has gone into the aerodynamics of the Mirage resulting in one the most aerodynamic vehicles in its class. The overall shape has been optimised to fine tune its drag coefficiency to just 0.27 and the sleek body, clam-shell bonnet and tapered roofline create the striking profile of a car engineered from the ground up for fuel economy and CO2 efficiency. (Source)
Profile taper of the roofline down toward the rear Drag-reducing "Kammback" type spoiler/roof extension (some models) Partial lower grille block off (passive -- size of blocked area varies depending on which transmission is equipped: the CVT car has a larger opening than the manual transmission model) Vestigal upper grille opening Relatively smooth wheel covers (some models), and narrow width tires (165 mm). The 175 mm width tires with open spoke alloy wheels on some Mirages result in worse fuel consumption, partly for aerodynamic reasons. Optimized rocker panel design (some models) Sharp trailing edge/corner on the rear bumper Front wheel spats/deflectors
(Also see thread: Aerodynamic wheel spats & Mirage)
Underbody smoothing panels Rear track 15mm narrower than front (permits plan taper in the bodywork towards the rear)
Of course, reducing aero drag isn't just about improving fuel economy. It also results in a quieter cabin (less wind & engine noise) and more engine power in reserve at highway speeds (faster high speed acceleration).
See also: Estimating Mirage gas mileage: May be highest MPG in its class in U.S./Canada
Mitsu provided technical drawings in some of its brochures which we can use to derive frontal area, and ultimately CdA. That figure is more useful for comparing the car's total drag area to values known for other cars.
Could the Mirage's drag coefficient be made even better?
Yes, at a cost (either financial, or affecting driveability). Here are a few drag-reducing modifications that already exist on other production vehicles that could be adapted to the Mirage:
- variable automated grille blocking
- reduced ride height
- additional underbody panels
- a larger "Kammback" drag reducing spoiler
Don't aerodynamics only matter at highway speeds?
People often incorrectly assume that aerodynamic design elements "only matter above 60 MPH" (or some other arbitrary high speed). The fact is, many vehicles burn more than half their fuel to overcome air drag at speeds as low as 25-30 mph (40-50 km/h), including the Mirage at Cd = 0.30.
So, yes, aero is critically important to highway fuel economy. But it also affects city numbers.
Of course the effect is exponentially greater the faster you go. When you double your speed, you cube (x8) the power needed to overcome aero drag.
EDIT: See post #13, below, for calculations that show a difference of 5 MPG between a Mirage with a Cd of 0.27 vs. a Mirage with a Cd of 0.31 cruising a speed of just 50 mph (80 km/h).