Posted Wednesday, August 2/06 in Mods & Tests

The fact that you're reading articles on a fuel economy web site says you're probably already aware that speed kills ... mpg. Resisting the urge to drive fast is one of the easiest and most effective methods for seeing big savings.

We've all seen stats from NRCan and the EPA about how much more fuel is burned as speed rises: Driving at 120 km/h instead of 100 uses about 20% more fuel (NRCan); observing highway speed limits can save 7-23% (EPA).

Those are serious numbers. But reading a stat is one thing - seeing it laid out graphically is a real eye-opener. So I set out to collect some hard evidence for the Blackfly: a graph of speed vs. fuel consumption.

Inspiration & examples ...

The inspiration for this data gathering mini-mission came from seeing a number of graphs for various cars, including this simulated plot for the Prius II. It's an interesting shape that reveals a lot about the car that made it:

(Source)

The benefit of electric assist is obvious up to around 42 mph, at which point the engine spins up, mileage abruptly falls off a short cliff, and then continues to decrease more gradually as speed rises. The shape of the graph could also have only come from a car with a continuously variable transmission - other than the transition where the internal combustion engine spins up, there are no obvious "steps" in the plot.

Note: that's a calculated graph, with values based on 68F, at sea level, no A/C or wind.

Next, here's a real-world plot of speed vs. mileage for a 1st generation Prius:

(Source)

While the cars these 2 graphs are based on do not share identical drivetrains, you can see similarities in the shape of the graph (particularly the dramatic change from assist to engine-on operation).

Non-hybrid examples ...

Here's one from the EPA for an unnamed vehicle (in unknown conditions) which shows a mileage peak around 50 mph - likely where the highest gear is engaged.

(Source)

Here's another with a bit more information about where it came from - a 1986 Volkswagen Golf GTI (though conditions are unknown):

(Source)

High speed mpg ...

A more extreme example of this kind of test was published earlier this year by Auto Bild magazine. They ran a number of cars at much higher (autobahn) speeds - between 50 and 140 mph on a test track while recording fuel consumption:

(Source)

The Blackfly graph: MPG vs. MPH ...

The purpose of this mini-experiment was simply to gather fuel consumption readings across a range of speeds to produce a graph. Only top gear was used. And these are the results:

Route & weather conditions ...

Conditions as of 3:00 pm EDT / 1900 GMT on 05/03/2006:

Air Temperature (ATMP): 58.8 F / 14.4 C *
Wind Direction (WDIR): NNE ( 30 deg true )
Wind Speed (WSPD): 3 kts / 3.5 mph / 5.6 km/h
Wind Gust (GST): 4 kts / 4.6 mph / 7.4 km/h
Atmospheric Pressure (PRES): 29.94 in

* (averaged with another land-based station)

1.7 km long runs on the almost level "test course". Each point on the graph is the average of a bi-directional run using cruise control (speed was set in advance of passing a start marker where the ScanGauge was reset).

Raw data ...

km/h ... W ... E - mpg (US) / L/100km / mpg (imp.)

 55 ... 83.1 / 2.8 / 99.8 ... 82.4 / 2.9 / 99.0
 60 ... 77.8 / 3.0 / 93.4 ... 76.3 / 3.1 / 91.6
 65 ... 74.1 / 3.2 / 89.0 ... 71.7 / 3.3 / 86.1
 70 ... 71.2 / 3.3 / 85.5 ... 70.8 / 3.3 / 85.0
 75 ... 67.2 / 3.5 / 80.7 ... 62.7 / 3.8 / 75.3
 80 ... 63.3 / 3.7 / 76.0 ... 59.3 / 4.0 / 71.2
 85 ... 59.4 / 4.0 / 71.3 ... 56.4 / 4.2 / 67.7
 90 ... 55.4 / 4.3 / 66.5 ... 53.9 / 4.4 / 64.7
 95 ... 51.5 / 4.6 / 61.9 ... 50.3 / 4.9 / 60.4
100 ... 46.6 / 5.1 / 56.0 ... 46.9 / 5.0 / 56.3
105 ... 44.9 / 5.2 / 54.0 ... 44.0 / 5.4 / 52.8

Modifications present at the time of this test included: grille block, rear wheel skirts, smooth wheel covers, tires @ 48 PSI.

Observations ...

• The first thing this graph confirmed to me is that it's much more difficult to exceed the Firefly's EPA highway fuel economy rating in highway driving than any other car I've driven. Which is to say: the speed at which it achieves its rating - 97 km/h (60.3 mph) - is significantly slower than other cars.

  I used to be able to set the cruise control in my 1989 Honda Accord at 95 km/h (59 mph) and exceed the car's highway EPA rating by about 20%.

  On the same day I ran the Firefly test, I took a relative's 1997 Toyota Camry automatic down the same test road; it achieved its 30 mpg EPA highway rating at 115 km/h (71.5 mph). Quite a difference.

• The even more optimistic NRCan highway rating of 66 mpg(imp.) / 55 mpg(US) is achieved at even lower speeds - about 89 km/h.
• The slope of the line sometimes surprises people - but it's important to note the Blackfly test is in top gear like the Auto Bild experiments, not through all the gears like the EPA and GTI graphs show.

Finally, what's the message these charts all send? For the best fuel economy, drive as slow as practical in the tallest gear.

Resources ...

• Most efficient speed for best MPG? - PriusChat.com
• 2003 Prius (Classic): MPG vs MPH - owner's web site
• Energy Efficient Transportation for Florida - Danny S. Parker, Florida Solar Energy Center (FSEC)
• Fuel Consumption at Higher Speeds - AutoBild - Green Car Congress

Related ...

• Showdown: testing RPM vs. MPG at a fixed speed - MetroMPG.com
• The government lies, and my car is fat - MetroMPG.com