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.

Plug-in Blackfly: going alternator optional nets +10% mpg

Posted Monday, October 23/06 in Mods & Tests

Alternator optional

How much energy does a car's electrical system use? Would shifting the electrical load from the gasoline-driven alternator to just the battery return a measurable fuel economy improvement? Will a modern computer-controlled car even run properly off its battery alone? Inquiring minds want to know!

My curiosity about this topic was piqued when I read about a guy who installed solar panels on the roof of his VW bus, added a couple of deep cycle batteries, and ditched his alternator. Apparently, the panels and the extra batteries were enough to drive round-trip from Texas to Seattle.

So, I tried it (minus the solar panels on the car). And it turns out that not only does running without the alternator result in a real MPG gain, it turned out to be the largest single fuel economy improvement of all the mods I've tried to date. But there's a big caveat: the immediate fuel savings may not make financial sense in the long run when you consider the effect on battery life.

Unbelted alternator overview ...

  • Smart alternators: old idea
  • Estimating electrical load
  • Estimating fuel needed to run the alternator
  • Unbelting the Blackfly: the water pump conundrum
  • Experiment: testing an unbelted alternator
  • Results & observations
  • VW verification: corroboration by another tinkerer
  • The BIG caveat
  • Alternative alternators: future designs increase MPG
  • Resources

Smart alternators: old idea ...

Before undertaking this little experiment, I had little doubt that alternators exact some fuel consumption penalty - the question was just how much. It's not hard to find examples where alternator use is minimized or abandoned entirely, either to maximize engine power or reduce fuel consumption:

- Drag racers are known to run alternator-less (or switch them off) when racing, for example.

Civic VX
The super-efficient Honda Civic VX used intelligent alternator control.

- The high efficiency Honda Civic VX and CRX HF models reduce alternator charging under certain cirucumstances. The VX computer instructs the alternator to drop from 14.5 volts to 12.5 (ie. not charging the battery) when all of the following are true:

- Load below 10A
- Speed below 40 MPH
- Engine speed below 3600 RPM
- Coolant temp above 140 F
- A/C off
- Intake air temp above 65 F
- Brake switch off
- Fuel cut-off not active

In other words, the alternator is essentially "idling" when the load on it is low, the car is warmed up and being driven gently. Note that the alternator reverts to normal charging when braking or decelerating (fuel cut-off mode), which is like a simple form of regenerative braking found in modern hybrids and electric vehicles. (Thanks to Randy for the VX info.)

Estimating electrical load ...

So it's clear there's some fuel efficiency benefit to be gained by fiddling with the alternator. But how much?

To figure that out, we need to start by estimating how much electrical energy is required to run the engine and accessories. Fortunately for me, Randy has already done the work. He measured the electrical power required to run his '94 Civic EX:

Item...................@ 14.4v

Idling ............... 128w
Rev up to 3500 rpm: .. +35w
Heater fan setting 1: . 55w
.. Setting 2: ......... 98w
.. Setting 3: ........ 141w
.. Setting 4: ........ 189w
Marker lights: ........ 95w
Full lights: ......... 240w
A/C (heater fan
subtracted, but
incl. condenser fan):. 163w
Defroster: ........... 104w
Wipers 1: ............. 48w
Wipers 2: ............. 76w
Radiator fan: ........ 144w

With that info in hand, a back of the envelope calculation begins: around 163 watts for running the engine, ECU & fuel pump; marker lights @ 95 watts; cruise control at 30 watts (just a guess). For a total of about 288 watts.

Estimating fuel needed to run the alternator ...

The alternator isn't 100% efficient. In other words, the engine has to contribute more than 288 watts to generate 288 watts. According to Wikipedia, a modern alternator is between 50-62% efficient.

At 60% efficiency, it would take 480 watts of work from the engine to generate 288 watts of electricity.

Back of the envelope calculations
Back of the envelope calculations: 530 watts (0.64 hp) of energy from gasoline to run the alternator under light load.

But wait, there's more!

Let's not forget losses to belt friction and aerodynamic drag at the alternator pulley fan (which, being smaller than the crank pulley, spins much faster than engine RPM). I'm not sure if pulley drag is figured in the Wikipedia efficiency calculation or not, but since this is a back-of-the-envelope exercise, we'll say it isn't. Let's say belt & aero losses consume an additional 50 watts.

So the back of the envelope tells us that my car requires around 530 watts, or nearly two thirds of a horsepower (745.7 watts = 1 hp) to run its alternator (for the load specified).

I've calculated elsewhere that the Firefly has to generate 5.4 hp to maintain 70 km/h in ideal circumstances. So reducing that power requirement by 0.64 hp should reduce fuel consumption by a similar amount: which is 11.9% (at that speed).

Worth trying? Sure!

Unbelting the Blackfly: the water pump conundrum ...

To do this test, removing the belt that turns the alternator was easy. Unfortunately, that belt also turns the water pump.

Of course, I could have just disconnected the alternator electrically and left the belt in place, but I wanted to do the full Monty and shed belt & pulley fan losses too. This required a bit of MacGyvering to get the water pump spinning again:

Inner tube: water pump belt material

Panty hose? No...

The first approach I tried (and the one used for this test) was to make 2-ply loop from a 3/4 inch wide strip of rubber cut from an old bicycle inner tube. I cut & stapled one together, stretched it taut over the crank & water pump pulleys, and surprisingly, it worked (long enough to test with, anyway).

Experiment: testing an unbelted alternator ...

- Weather

June 7, 2006 - 8 PM
Temperature ... 12.3° C
Pressure ...... 1011.2
Wind .......... NNE 4.9 kts

- The experiment

This was an A-B-A comparison on my usual nearly level test route, where A = alternator connected as usual, and B = alternator belt removed (with the temporary rubber belt running the water pump from the crank pulley).

Speed was 70 km/h (43.5 mph) using cruise control set once, cancelled with the brake between runs, and resumed for subsequent runs.

- Results

@ 70 km/h / 43.5 mph - MPG (US)

A = alternator connected
B = alternator disconnected (belt off)

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

A ..... 71.2 ... 68.1 ... 71.65 / 3.28 / 86.05
A ..... 73.4 ... 68.7 ... 71.05 / 3.31 / 85.33
A ..... 73.1 ... 68.5 ... 70.80 / 3.32 / 85.03

B ..... 78.1 ... 76.3 ... 77.20 / 3.05 / 92.72
B ..... 81.3 ... 75.4 ... 78.35 / 3.00 / 94.09
B ..... 82.3 ... 75.1 ... 78.70 / 2.99 / 94.52

A ..... 75.0 ... 66.6 ... 70.80 / 3.32 / 85.03
A ..... 72.0 ... 67.2 ... 69.60 / 3.38 / 83.49

A average ... 70.78 mpg (US) / 3.23 / 85.0
B average ... 78.08 mpg (US) / 3.01 / 93.77

7.3 mpg (US) - difference of between A avg. and B avg. runs
10.3% - increase in fuel economy without alternator

Chart: alternator vs. no alternator


  • The last group of A runs were made after reinstalling the belt and driving for 10 minutes to replace energy taken from the battery from the B runs (simply re-installing the belt and doing the final A runs would have skewed the results as the alternator would have been under more load while recharging ).
  • Electrical loads, all runs: cruise control, parking lights
  • For the last bi-directional A run, the headlights were on (ran out of daylight). This added electrical load can be seen in the run's slightly lower numbers.

Observations ...

  • Wow! What a surprise - 10% is significant. In fact, this turned out to be the largest single fuel economy improvement of all the mods I've tried to date.
  • Smooth move: with the alternator unbelted, the engine seems to run smoother and is more responsive, particularly at low rpm where I usually have it - but that would make sense for a small engine in a light car where the electrical load is a proportionately larger portion of driving load.
  • Long-term success: Since doing this test, I've driven the car for more than 400 cumulative km with the alternator disconnected, recharging the battery either from from the grid (using a standard battery charger) or solar panels.

    Water pump belt: v. 2.0
    Water pump belt: v. 2.0

  • Water pump belt v.1 and v.2: the main problem encountered was the MacGyvered belt I made to run the water pump. The double-ply loops cut from inner tubes and stapled into shape was troublesome - they would only last 50 or 75 km before tearing (usually from one of the staple holes) and flying off. I ended up making a stouter version from a length of bungee cord, with the two ends lashed (siezed, actually) tightly together with dental floss. That version has worked much better - it's been on the car for 250+ km with no trouble.
  • How far on 12v? On one nearly 50 km / 30 mi trip @ 14 C / 57 F, in a combination of city & rural secondary roads,(30 mph avg, 45 max.) battery voltage was down to 12.4 near the end (after letting the battery rest for 10 minutes). That means roughly 20-25% of its capacity was drained, assuming a 12.6 v start. This was a very low load trip - no lights/heater blower, even relatively little brake/signal lights. In recent colder weather and with more accessories on, I'm getting less than half that "range": e.g. 34 km from 12.6v to 12.2v, with headlights on part of the time and the heater blower on low for the duration.
  • Sagging volts: one of the biggest drawbacks of removing the alternator, is that the voltage of the electrical system is no longer stable at around 13.5 volts. It starts at battery voltage - 12.6 (assuming a full charge) - and falls as more load is placed on it and as it discharges. Unfortunately this means degraded electrical performance. I avoid driving "belt-less" at night because I want bright headlights. Also I have felt the engine "stumble" on a few occasions when I let the battery voltage get too low (low-11.x volt range, under load).

  • Solar concept: theoretically, you could run a car's electrical system on a sunny day entirely with solar panels , but the cost would be prohibitive. Even leaving the alternator in place and using solar panels would save fuel (assuming the alternator was configured to back off when the panels were supporting the electrical load).

VW verification: corroboration by another tinkerer ...

Skeptical of my claims? Well, you don't have to take my word for it!

Fellow efficiency enthusiast Brock did a similar test with his 2003 VW Jetta TDI, and reported similar results, though he had to be a bit more creative in his approach, since the TDI draws a lot more current when cold (glow & coolant plugs), and the engine computer was unhappy when the volts sagged too low:

"I have found I get about a 5-7% increase in mileage with the alternator disabled. I leave the alt in there and just pull the main fuse so it basically sees no load. I am sure if I removed the alt or belt it would help a bit, but if my wife takes the car to work I can just pop the fuse back and everything is back to normal.

Brock's Jetta TDI
Brock's sometimes alternator-less Jetta TDI saw a 5-7% fuel efficiency gain.

The car will "throw codes" because it sees something is wrong. So I had to find a way to keep the main battery topped off.

So what I did was took some parts I had laying around, 2 6v deep cycle batteries (golf cart) put them in series behind the passenger seat. I ran that in turn to a 1500w inverter which in turn feeds an iota DLS 55 amp charger set at 14.4v. The voltage will slowly drop until the coolant heaters shut off then quickly come up to 13.8v. I haven’t thrown a code since I tried this setup.

I originally tried just paralleling the batteries but the voltage under those loads would just drop too far, usually about 12.4v and the car didn't like it.

But getting back to my testing. I ran a set route, 3 miles there and back 6 times, 3 each way. On my test runs I got 87 mpg with the alt on and 92 mpg (according to scangauge) with the alt fuse pulled - about 5% better. I ran a whole tank last spring and saw about a 5% improvement on the entire tank as well.

The neat thing also was with extra batteries at home, I always had at least two charged and ready to go for my next trip - charged off solar panels as well."

Thanks for the corroboration, Brock!

The BIG caveat: batteries are consumables ...

At first blush, the 10% gain makes this seem like a no-brainer mod. But it may not actually make sense from a strictly financial point of view. You could end up spending more than you'll save, particularly because a battery is a consumable that wears out relatively quickly when discharged deeply and repeatedly.

The big caveat: battery life
The big caveat: battery life

A conventional "starting" battery is not at all suited to this kind of punishment. I have read that you may only expect to get 50 deep discharges from a starting battery before it's damaged beyond usability. To use this mod, a more durable flooded or deep cycle battery is definitely required.

Some really rough calcs, using my own situation:

If I had saved 10% by running without the alternator over the past year, then my fuel bill would have been reduced by roughly $34 over 8,700 km, or 0.39 cents/km. (And that's at higher Canadian fuel prices.)

Assuming I get 200 cycles from a deep cycle battery before it's toast, and can drive about 75 km before it needs recharging, that's 15,000 km per battery. The battery cost (assuming $90 per) is therefore 0.6 cents per km.

So already I'm "paying" for my increased efficiency (0.6 cents out, 0.39 cents saved), and I haven't even factored in the cost of electricity yet (which is admittedly small where I live). Unless you happen to have a source of cheap or free batteries, you need to do the math before jumping on this mod as a way to save money.

On the other hand, if I look at it from an emissions perspective, it's a different picture: 2/3 of Ontario electricity is GHG-free (1/3 nuke, 1/3 hydro). That's arguably better than using gasoline to generate my car's electricity.

Alternative alternator ideas ...

Recognizing the fuel consumption penalty of conventional charging systems, a number of companies are working on alternatives:

  • BMW is developing a "smart" charging system and claims it reduces fuel consumption by about 4%. Its basics are similar to the Civic VX example (above), with the main difference being a generator integrated in the braking system for regeneration. (Source: Green Car Congress)
  • Engineers in a British organization are working on an exhaust gas-driven turbo generator that, under the control of the engine management system makes use of exhaust pressure to charge the battery when appropriate (e.g. at higher RPM) and permits the car to draw energy from the battery when the engine is under low loads. (Source: Green Car Congress.)

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