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.

Kardboard Kammback: testing a partial boat tail prototype

Posted Saturday, September 9/06 in Mods & Tests

literal boat tail

One of the biggest aerodynamic problems with the majority of hatchback-shaped vehicles (like my Blackfly) is the size of the trailing wake. The large reduced pressure zone acting on the rear end can account for up to 1/3 of total drag.

That said, a hatchback configuration isn't automatically an aerodynamic death sentence. On the contrary, three of the most slippery production cars available in recent years were hatchbacks: the 2nd generation 2004- Toyota Prius (Cd .26), 1999-06 Honda Insight (Cd .25), and 1999-05 Audi A2 (Cd .25).

Prius, Insight, A2
Toyota Prius, Honda Insight, Audi A2

But these cars are exceptions to the rule. So what do you do if you're stuck with a conventional more or less van-shaped hatch? Just grin and bear the aerodynamic shame of it? No way! Get out the cardboard and duct tape, and go to work fixing what the stylists messed up!

Boat tail prototype overview ...

  • hatch wake: size matters
  • teardrop shape reduces wake
  • chopped teardrop: the Kammback
  • boat tail retrofits: a known cure
  • kardboard Kammback prototype
  • testing the partial boat tail
  • observations
  • links & resources

Hatch wake: size matters ...

I've only ever found one reference to the drag coefficient (Cd) of the 1995 and up Geo Metro, quoted at .34. That's not good, but it's believable, given the car's profile. It's kind of got a van-shaped rear end, where the air flow separates near the maximum roof height, causing the size of the trailing wake to be practically the same size as the vehicle's projected area.

Modern sedans - "3-box designs" - with trunks of sufficient height and length, normally have less drag than their "2-box" hatchback counterparts. This is easy to understand when the relative size of their wakes is revealed by a smoke test in the wind tunnel:

Wind tunnel: Golf vs Jetta
Wind tunnel test showing the relative wake size of a VW Golf (Rabbit) hatchback and Jetta sedan

Teardrop shape reduces wake...

Solar car streamlines
Ideal tear drop shape common tp solar race cars showing turbulent (blue) and separated (red) air flow.

Aerodynamicists have known for a long time that minimizing the size of the wake decreases Cd and increases fuel economy. The shape that best achieves this is one which tapers at the rear in the approximation of a teardrop, both in profile (roof tapered down and floor tapered up toward the rear ), and in plan view (as seen from above, sides tapered back toward the centerline).

By keeping flow attached as long as possible, adverse pressure gradients are avoided and streamlines are shepherded gradually together behind the vehicle resulting in minimum pressure drag.

Chopped teardrop: the Kammback

Obviously, it's not practical to use a fully tapered teardrop shape in an every day vehicle - the back end would be too long, and would have limited usefulness for carrying people and cargo (one of the advantages of the van-shaped hatchback).

Fortunately, in the 1930's a German aerodynamicist named Wunibald Kamm discovered that when the ideal teardrop taper was applied to the shape of a car body, it could be cut off well before the tip with little change in drag - the air flowing off the rear still behaves as if the full teardrop shape is there. The Kammback was created.

Not surprisingly, all three of the low-drag hatchbacks shown above employ the Kammback design.

Boat tail retrofits: a known cure ...

Transport boattail

"Fixing" a poor rear end design by retrofitting aerodynamic add-ons to minimize wake isn't a new idea.

Much attention has gone to studying and reducing the size of the wake behind large trucks, since their cargo-carrying capability dictates a boxy profile with no taper at all. Researchers have known for decades that:

Aerodynamic drag can be significantly reduced with trailer add-ons that reduce the wake and increase the base pressure. - Source: SAE 2000-01-2209

Kardboard Kammback prototype ...

Mark's Metro tail

While the seed of inspiration for trying something similar on the Blackfly was planted originally by the EVWorld article about Phil Knox' aero-modded Toyota pick-up, the final push to get out the cardboard, box cutter and duct tape to construct a drag-reducing shape came from a drawing made by Mark and posted on his web site (now defunct).

(I know, I know. Technically it's not a Kammback, since it doesn't quite reduce the rear area of the car down to 50% of the total projected area... I just like typing 'Kardboard Kammback'.)

To make a long story short, I adapted Mark's basic design in about 45 minutes, with the goal of simply extending the existing roof & side taper already designed into the car.

And despite the impression that the boat tail must obstruct vision out the rear window, the horizon was actually still visible in the rear view mirror, just below the end of the cardboard.

It sure looked interesting. But did it work? Quick! To the wind tunnel!

Testing the Kardboard Kammback ...

partial boattail cardboard prototype 1

Much to my neighbours' entertainment, I left the cardboard creation on the car long enough to wait for a calm day to visit my usual test route for some before & after runs. It's always a risk making car parts out of cardboard: had it rained, the boat tail would have turned to mush and fallen off. Fortunately, I beat the weather!

I wouldn't call this a rigorous test: Cardboard/duct tape prototypes don't lend themselves to multiple removal & re-installations, so the preferred A-B-A method was not used.

That said, conditions were great:

- Weather

Observed at: Grenadier Island 18 July 2006 3:00 PM EDT
Temperature ... 29°C
Pressure ...... 101.5 kPa
Humidity ...... 62 %
Wind .......... SSW 4 km/h

- The experiment

This was a simple A-B comparison, where A = with cardboard boat tail prototype, and B = cardboard removed.

partial boattail cardboard prototype 2

Speed was 55 mph (88.5 km/h) using cruise control, and it was hot, so the fan was on 3/4.

- Results

@ 88.5 km/h / 55 mph - in MPG (US)

A = boat tail on
B = boat tail off

A/B ... West ... East ... Average MPG (US) / L/100 km / MPG (imperial)

A ..... 59.8 ... 61.6 ... 60.70 / 3.88 / 72.9
A ..... 60.2 ... 63.3 ... 61.75 / 3.81 / 74.16
A ..... 59.4 ... 63.1 ... 61.25 / 3.84 / 73.56

B ..... 57.2 ... 61.7 ... 59.45 / 3.96 / 71.4
B ..... 58.7 ... 61.8 ... 60.25 / 3.90 / 72.36

61.23 mpg (US) - average of A runs
59.85 mpg (US) - average of B runs

1.38 mpg (US) - difference of between A avg. and B avg. runs
2.3% - increase in fuel economy with the boat tail

Observations ...

Interesting! On the one hand, I was pleasantly surprised to see a measurable result from a slapped-together prototype. But at the same time, I was also slightly disappointed that it wasn't a larger effect.

partial boattail cardboard prototype 3

The difference between this mod and the mirror removal, grille block and wheel skirts, is this is a complex shape that needed to be planned with sound theory. Unlike those other aero mods, which were essentially no-brainer parts-removal and gap-fillers, this one had the potential to actually increase drag if not shaped properly.

I'll probably attempt to refine it a bit more. I paid fairly close attention to getting the top right: I stayed at a fairly conservative 10 degree taper, where the Prius and A2 are slightly more aggressive with angles around 12-13 degrees. Also, I didn't pay enough attention to the sides - they were different angles, and also the side/top transition should be radiused, not sharp.

The next version should also be made of something a little more durable than cardboard (I'm thinking foam core & fiberglass). One of the things that interests me most about making a permanent rigid "hatch tail" is that it could serve as a mounting point for a more extreme, extended version of itself - something to get the car even closer to that ideal teardrop shape for highway use.

But I think before I do that I'll move on to other mods first which - based on what I learned here - may have the potential to yield bigger results (smooth undertray comes to mind).

Resources ...

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