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.

Showdown: testing RPM vs. MPG at a fixed speed

Posted Monday, March 13/06 in Mods & Tests

Firefly torture test: 5100 RPM in 2nd gear!

If you row your own gears (as most fuel economy nerds do), here's a mini-experiment that should be of interest.

I recently did a test run that nicely illustrates the fuel efficiency "costs" of reciprocating mass, accessory drag, and internal engine & transaxle friction at varying engine RPM.

If you ever doubted the economy driving tip that says you should be in the highest practical gear at the lowest possible cruising speed, doubt no more.

The experiment:

Simply, I recorded fuel consumption in four different gears (2nd through 5th) at the same road speed (60 km/h, or about 40 mph) on the same stretch of road.

Route & weather conditions:

The test route was a 1.8 km / 1.1 mile long, very slightly rolling road (maybe a 5-10 ft elevation change).

Air Temperature (ATMP): 18.0 �F / -7.8 C; Feb 28/06; 3:00 - 4:00 PM

Wind Direction (WDIR)...WSW ( 240 deg true )
Wind Speed (WSPD).......22 kts (40.7 km/h / 25.3 mph)
Wind Gust (GST).........25 kts (46.3 km/h / 28.8 mph)

Wind note: I probably wasn't subjected to the full extent of the wind indicated above. The weather station I get my readings from (located roughly 5-10 km from the test route) is fully exposed in open water on the St Lawrence River, and the 1.8 km stretch of road I used has portions which are tree-lined, and other portions which are protected by some land elevation to windward.


All readings were taken going in the same direction (SW); I looped back around for each run. The car was warmed up prior to this test with about 30 minutes of errand running (in-town driving).

graphic: Gear vs MPG - Firefly & xB

Test speed: 65 km/h / 40.4 mph (+/- 1 mph)

ScanGauge readings (avg MPG function)



For reference: going in the opposite direction (NE) along the same 1.8 km route at the same speed:


(Nice illustration of the aerodynamic penalty/benefit of wind, eh?)

After posting my results, a fellow fuel economy enthusiast did a similar run in his ScanGauge-equipped Scion xB and came up with these figures:

Test speed: 38 mph / 61.2 km/h




Chart: RPM vs MPG in my '98 Firefly

  • Take this as "snapshot" FYI info only. The precision is probably not very reliable due to the small sample, short course length, uni-directional (non-averaged) runs, and I didn't use cruise control.
  • Oh, the abuse! I hated doing that 2nd gear run. The Blackfly isn't equipped with a tachometer, and without the ScanGauge's digital tach I would have sworn I was on the verge of seeing pistons flying through the hood! Fortunately, I knew ahead of time that the rev limiter activates at 7000 RPM, so I had revs to spare (according to my factory service manual). Sheesh - the things I do for science.

Practical implications:

  • The old adage appears confirmed: when cruising, use the highest gear that's practical for your road speed.
  • This test reveals the possible advantage of using larger circumference tires/wheels to further reduce engine RPM for a given road speed. The Blackfly is equipped with 13 inch tires, and at 155/80/13, they're already the "tallest" commonly available 13 inch size. So I would have to change to a larger rim to get a larger circumference tire.

    Using this tire calculator, my stock 13 inch wheel & tire has circumference of 71.5 inches (circumference = pi * diameter). Upgrading to a 185/65/14 tire with a circumference of 73.73 inches, would net an increase of 2.23 inches / 3.1% more than stock.

    How much fuel would that save me? Well, during the test, dropping the engine speed by 420 RPM (4th to 5th gear), or 16.8%, raised mpg by 8.9 mpg (47 to 55.9), or 18.9%. So switching to the 3.1% larger circumference wheel & tire would reduce RPM by the same percentage (3.1%). That would theoretically increase MPG by 3.1% / 16.8% * 8.9 mpg = 1.64 mpg (US) in top gear.

    (Note - assumes no additional penalty from: increased aerodynamic drag from the wider tire and taller ride height; possibly increased rolling resistance; more rotational mass; AND assumes and that the relationship between the change in RPM and MPG is linear.)

  • What about a different transmission? Vehicles that are sold with different engine sizes (but with the same number of cylinders) often use different gearing in their various transmissions. Yet their transmissions are often physically interchangeable.

    For example, over the years, the Suzukiclones had 3 or 4 interchangeable transmissions with different gearing due to the range of engines available across the line: from the lowest power, ultra-efficient 3-cylinder XFi; to the garden-variety 3-cylinder; to the turbocharged 3-cylinder; the garden-variety 1.3L 4-cylinder; and the GT/GTi twin-cam multi-valve 4-cylinder. Honda's line-up tells a similar story.

    I'd be lying if I said I haven't been toying with the idea of trying out a taller transmission from the 1.3L car in the Blackfly at some point.

  • Of course it must be said that messing around with gearing, whether directly via a transmission swap or indirectly by increasing tire diameter, will affect acceleration. I have heard first-hand and second-hand about two different Metro owners who made transmission swaps to "taller" gearing and were unhappy with the results (reduced acceleration performance).


- Auto Tire Diameter Calculator (P-metric)


Update: March 27, 2006...

Feedback from John G, originally posted at related to my thoughts on getting a taller transmission (and my assumption that the relationship between MPG and RPM is linear):

"Efficiency and engine RPM are not directly related, meaning that a 25% RPM reduction does NOT predict an efficiency improvement of "x%", nor does it predict any improvement at all - although RPM reduction will generally improve efficiency.

[Your] data proves this out. At first [you] get a 25% increase in RPM, but only a 15% MPG change (about 50% of the RPM change percentage). Later [you] test with a 48% RPM increase, but see a 60% reduction in MPG (150% of the RPM change percentage).

The efficiency of an internal combustion engine is largely influenced by operating RPM, BUT there is one "sweet spot", typically at or near "Peak Tq" that will return the best efficiency.

... I had a Bronco that got better mileage at 60 MPH than it ever did at 55 MPH, which certainly does not make sense based on the fact that the air drag at 60 is 112% of that at 55 mph, and the RPM is 10% higher as well."

Thanks for the comments, John.

Incidentally, I've seen evidence of the "sweet spot" in regular driving (without attempting to test for it properly - though it's on my list): when I switch into top gear, my "instant" fuel economy often continues to improve for a short while as I gradually increase RPM and build up speed.

Not only does that confirm John's point that MPG is not directly related to RPM, it's also evidence that fuel efficiency can be worse if RPM is kept too low.

That said, at least on my car, even when driving the engine at below optimal RPM, choosing a higher gear still results in higher fuel efficiency than the same road speed in a lower gear.

On the question of whether to go with bigger wheels/tires or a taller transmission, Rick writes:

"My vote is for the larger tires. If you check the specs area at can get some exact tire weights and so forth. Also wheel weights. Would really like to see you test the larger tire idea. I have a set of 2% oversized tires in storage...waiting until needed to install them."

If only the Metro/Firefly didn't have an uncommon bolt pattern, it would be relatively easy for me to borrow a set of larger rims to try this out. Unfortunately, I'm stuck with bolt pattern not used by Honda, VW, Toyota, etc. But if the opportunity comes up, I'll certainly go for it!

Related ...

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