My Pontiac Firefly / Chevrolet Metro / Geo Metro / Suzuki Swift welcomes fuel efficiency nerds everywhere

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Latest fuel economy stats
for my '98 Firefly 1.0L 5-speed
  best: 2.3 125.1 104.2
 worst: 6.4  44.1  36.8
prev.3: 3.3  82.3  68.6
   all: 3.8  73.4  61.1
L/100km | mpg IMP | mpg US
Jul 28/07: more, graph, calc.
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Best non-hybrid MPG: Mitsubishi Mirage
Highest MPG for a new car: Mitsubishi Mirage?
Mitsubishi's 1.2L, 3-cylinder Mirage is the first new non-hybrid car that can match an old Metro's mileage. The company says 44 mpg (US) highway, 37 city. (Some drivers are already beating that in various economy driving contests.) How? An efficient engine, very light weight and aerodynamic design.

Cheapest to own? 2015 Nissan Micra Forum
2015 Nissan Micra Forum
The Micra's fuel economy isn't its most notable feature -- the $10,000 price is. That makes it one of the cheapest cars to own. And its 109hp, 1.6L engine and good power-to-weight ratio means it's fun to drive too.

Latest 10 posts:
1. Recipe for getting 99.7 mpg from a Geo Metro
2. - famous aerodynamic Honda Civic gets a web site
3. Snapshot: effect of tire pressure on rolling resistance
4. 65+ vehicle modifications for better MPG
5. Metro mania: forget stocks, put your money in old Geos!
6. 100+ Hypermiling / ecodriving tips for better gas mileage
7. Experiment: how long should a block heater be plugged in?
8. Everything old is new again: Car and Driver magazine modifies an econobox to improve MPG
9. Project Convertible XFi: alfresco efficiency
10. The floor is yours: MetroMPG opens a fuel efficiency forum
11 ... 64. Show all posts

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Good MPG forums: I spend a lot of time at and have also been known to lurk around

Chevrolet Aveo forum - discussion of the Chevrolet Aveo and its siblings (Pontiac Wave, Pontiac G3, Suzuki Swift+, Daewoo Kalos).

> Lots more Metro links...
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Send me a note:
darin AT metrompg D-O-T com,
or here

MetroMPG has opened a fuel economy forum
Read about the project here, or go straight to
ScanGauge fuel economy computer Save fuel with a ScanGauge II fuel economy computer.
I personally recommend this tool. I've owned both versions (I and II) and can't say enough good things about it. If you're serious about saving fuel, get one.

For more information and to order, visit EcoModder.

Testing grille blocking & wheel skirts: +5.7% improvement

Posted Tuesday, December 6/05 in Mods & Tests

Blackfly with aero mods

The start of a conversation I had this weekend at a Christmas party:

Neighbour - "So, looks like you got a new Firefly."

Me - "Yup."

Neighbour - "Sort of looked like there was some cardboard taped on it."

Me - "Um... Yup."

I was just relieved he didn't notice last weekend, when I pulled into the driveway with wet newspaper plastered across the front of the car. Somehow that fact made it slightly less unusual to be explaining homebrew aerodynamic mods at a Christmas party. It was made even easier still by the fact that the modifications were a solid success.


Overview: testing DIY aerodynamics:

  • aero inspiration
  • the mods: grille block & rear wheel skirts
  • test route & conditions
  • test results at 95 km/h (59 mph)
  • observations & conclusions
  • future aero tests
  • other resources


Aero inspiration

This all started when I came across an EV World article about a guy called Phil Knox, who aerodynamically modified his 1994 Toyota pickup and boosted his highway mileage by 28%. While his modifications were fairly extensive, they weren't difficult to make - with materials available from any home improvement store.

the Phil Knox modified Toyota T100

Tongue-in-cheek, the EV World article described Knox as a "preacher" spreading "the environmental gospel of aerodynamics".

And clearly, I'm not the only convert. Read the comments following that article and you'll see that Phil Knox has inspired more than a few folks to learn about aerodynamics and efficiency. Those extensive and enthusiastic comments apparently vaulted the story to "most-discussed" status at EV World. It even resulted in the launch of a Yahoo group to continue the dialogue in a more user-friendly forum.

And ultimately, it resulted in the article you're reading right now.

Keep on reading to find out how my first steps into DIY aerodynamics increased my highway mileage by nearly 6% under controlled conditions.

grille block - click to zoom
The tiny, offset radiator is visible behind the unblocked openings.

The mods: grille block...

If you read about the one-shot aerodynamic test that was an "appetizer" to this one, you'll recall my roadside modification took the form of a "papier mache" wet newspaper barrier to divert excess airflow around the car which would otherwise have gone through the grille, the radiator and the engine compartment. In addition to aero benefits, the grille block aids efficiency in cold weather by permitting the engine to reach operating temperature sooner, and the transaxle to run warmer also.

The results, while startlingly good (an apparent 3.1% mileage boost), were based on just a single run and so could hardly be called definitive.

But they were definitely enticing - I was encouraged to to have a second look, this time over repeated runs. So I remade the grille block using cardboard and duct tape. (I've graduated from papier mache!)

rear wheel skirt - click to zoom

... and rear wheel skirts

The second modification under examination was also very quick and easy to make. Rear wheel skirts - cardboard cutouts that enclose the back wheels within the wheel arches. Think Honda Insight.

The aerodynamic principle is simple enough: an uninterrupted air flow along the side of the car past the wheel skirts will be cleaner than one that has to cross the gap between the body and wheel, plus interact with the turbulence generated by the spinning wheel itself.

wheel skirt detail
The skirt had to hold a slight curve to clear the tire and wheel cover. Coat hanger wire was taped on the inside bottom edge and that allowed me to shape it.

These are easy mods: after 20 minutes with a utility knife, some cardboard and duct tape, I headed for the "test course".

Route, conditions & results

Again, this ScanGauge-enabled trial was run under constant speed-controlled conditions (using cruise control, set once), on a level stretch of road. Unless otherwise noted, results are averaged from bi-directional runs to cancel any effects of wind/grade. (For details of the course, see the air filter test, part 1.)

The weather was overcast and calm, with NE winds below 3 kts (5.6 km/h / 3.5 mph). The road was dry. The temperature was about 2 C / 35.6 F.

Just to review, here's the result from the one-run "appetizer" from before, testing the papier mache grille block:

  • @ 80 km/h / 49.7 mph --- West --- units: km/gal (US)

    Control * -------------- 75.78 W ------------------- 75.78 avg
    Grille block (GB) ------ 78.20 W ------------------- 78.20 avg
    - - -
    Grille block increase over control: 2.42 / 3.2%
    - - -
    * Control = average of 4 westbound non-aero runs

    I started this new set with a bi-directional run with the cardboard grille block; next was a series of 3 bi-directional runs with combination of grille block and rear wheel skirts:

  • @ 95 km/h / 59 mph --- West/East --- units: km/gal (US)

    Control * -------------- 72.67 W ----- 69.07 E ----- 70.87 avg
    Grille block (GB) only - 74.00 W ----- 71.40 E ----- 72.70 avg
    GB & skirts - run #1 --- 76.30 W ----- 73.60 E ----- 74.95 avg
    GB & skirts - run #2 --- 76.30 W ----- 73.90 E ----- 75.10 avg
    GB & skirts - run #3 --- 76.00 W ----- 73.40 E ----- 74.70 avg
    - - -
    Grille block only increase over control: 1.83 / 2.6%
    Average of GB & skirts bi-directional runs = 74.92
    Average GB & skirts increase over control: 4.05 / 5.7%
    - - -
    * Control = average of 7 bi-directional non-aero modded runs

    Observations & conclusions

    results chart

    • The grain of salt: the calculated margin of error in the 7 control runs: plus or minus 0.18 km/gal / 0.25% from the average.
    • The bi-directional grille block only run supports the results of the appetizer trial: 2.6% vs. 3.2% improvements, respectively. An average increase of 2.9%
    • The engine temperature remained stable across all the runs, blocked and unblocked. That tells me I may be able to block one or more of the remaining grille openings.
    • Together, the grille block and wheel skirt improved efficiency by 5.7%. So, the wheel skirts account for about 2.8% of that.
    • From a hypothetical baseline of 50 mpg (US) for the Firefly, this translates into a total boost of 2.9 mpg.

    It looks like it's time to graduate again - from cardboard to something that won't turn to mush in the rain and snow!

    Making a more durable grille block will be easy: either a vinyl bra, or semi-permanent plastic slats installed in the grille openings would do the job. I say semi-permanent because I still want the option of easily changing cooling flow as the season and other conditions demand.

    It should go without saying that tinkering with grille blocking requires careful attention to the temperature gauge to avoid potential overheating.

    Wheel skirts will be more challenging, as they need to look respectable and also be easy to remove (to facilitate tire changes or other repairs that require the wheel to come off). I'm also reluctant to drill any holes in the bodywork for fasteners, as they will be prone to rust.

  • One final observation: tinkering with aerodynamics is bound to draw attention - like comments at a Christmas party. Aero experiments will yield more than just efficiency data - you'll get psycho/social feedback as well:

    "That look -- especially the distinctive 'skirt' that extends halfway down the rear wheels -- is not for everyone. When I took the Insight for a spin, one driver stopped me to call it a computer mouse on wheels. (Business Week article).

    Why exactly people can have such strong reactions to an aerodynamically shaped car is a topic for another post.

    Other possible future aero tests

    Tuft testing; wheel disks; deflectors/dams ahead of the tires; ride height lowering; front wheel arch gap fillers; underbody panels (belly pans)...


    Free Fuel Riding on the Wind - EV World

    MaxMPG Yahoo group

    Papier mache aerodynamics: +1.5 MPG? -

    Aerodynamics links - - (scroll down to Aerodynamics)

    EcoModder fuel economy forum Note: MetroMPG has opened a fuel economy forum
    Read about the project here, or go straight to

    darin AT metrompg D-O-T com, or here