Posted Tuesday, April 25/06 in Driving efficiently

Two records were broken lately: oil hit $75 US a barrel, and I squeezed 60.7 mpg (US) from my last tank of gas - a personal best.

(For those of you more fluent in other units, that's 72.9 imperial mpg, or 3.9 L/100 km.)

You'd be forgiven for thinking that the return to high gas prices and my recent fuel economy performance are related - but they're just a happy coincidence. In fact, for most of this tank - actually, before the latest spike in gas prices - I had been test driving a new driving technique. Well, really it's an old technique that I had stopped using and have now returned to with open arms... and great results.

A little MPG perspective

For this record-setting tank, I traveled 642 km / 398.9 miles in a mix of city & rural (slower) highway driving. Springtime temperatures ranged from 7 C / 45 F up to around 15 C / 59 F.

60.7 mpg (US) is...

• 14% above my previous best tank in this car (of 53.1 mpg (US) the previous fill-up)


• 32 % higher than the US EPA combined rating (of 46 mpg) for this car

It's high enough that I had to re-jig my fuel economy chart to be able to graph the newest figure! (The upper limit on the chart used to be 60 mpg!)

Gentlemen, stop your engines!

In a nutshell, the key is... to use my key! By that I mean, shut the engine off whenever possible and when safe: when stopped, and when coasting.

That's what I did for the majority (but not all) of this tank, and it was the most significant change from my previous fill-up (which at 53.1 was my previous best in the Blackfly). Spring weather was also more prevalent.

So why hadn't I been using this technique all along?

I used to do a fair amount of engine-off coasting in my 1989 Accord. I have even written about the "manual" auto stop feature before.

But after I got the ScanGauge, I let the habit wane because switching off the key to stop the motor in the Suzukiclone also puts the ScanGauge to sleep. And unfortunately for me, the Firefly uses the ISO ODB protocol, which compared to other (newer) protocols takes fairly long to re-initialize (roughly 15 seconds) when switching the key back on. The result of all this was seriously skewed fuel consumption data on the ScanGauge when using engine-off coasting on a regular basis.

So... I stopped doing much engine-off coasting. In retrospect, the irony is clear: in order to keep my fuel consumption instrumentation from being messed up, I abandoned a technique that was helping my fuel economy!

When another efficiency enthusiast alerted me to the possibility of using a kill switch rather than the key to shut off the engine (leaving the key on and the ScanGauge active), I leapt at the chance to see if it worked - and it did.

So it was time to find out just how much I had been missing by not coasting the Blackfly with its engine off. And holy oil prices Batman, I'd been missing A LOT...

Mini experiment: comparing MPG when coasting at idle vs. engine-off

I'll post the full details of this mini experiment in a separate, upcoming article.

But the executive summary is that on back-to-back loops of 100% city driving over the same route, where the only significant change was coasting when possible with the engine idling, vs. coasting with the engine-off, the engine-off technique resulted in 12.9% better fuel economy - 60.4 mpg (US) vs. 53.4 mpg (US).

The hybrid flip-flop

Since re-incorporating engine-off coasting into my regular driving, I've seen something very interesting happen to my city vs. highway fuel economy: it's become easier for me to get higher numbers around town than it is on the highway. In fact, in city driving, I've achieved fuel consumption figures for individual round trips (as reported by the ScanGauge) that are higher than can be accomplished at any constant (highway) speed.

This is similar to most hybrid cars' fuel economy ratings, which are higher in the city cycles than on the highway. That's not surprising, since many of these cars also employ engine "auto stop" features when decelerating and when stopped.

Bigger numbers on the horizon

I'm expecting even better numbers from future tanks:

• this 60.7 came from using engine-off coasting for only part of the tank


• ambient temperatures are rising again


• on three individual round trips last week, I achieved over 80 US mpg, according to the ScanGauge.

Before the summer's out, I'm predicting at least a tank or two above 70 mpg (US) and beyond. Good thing, since the price of fuel seems headed only upwards too.