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I used data gathered by MetroMPG on the Blackfly and the 06 Corolla to find the optimum speed to drive each car, based on the premise that saving gas is good, getting to your destination sooner is good, and depending on how much you value your time and your fuel, there is a correct balance. So I assigned a dollar value to time and to gas.
I pay something like $2/gal, but I feel obligated to save gas like it was much more expensive. I value it at $6/gal. Getting there sooner also matters to me, to the tune of about $12/hr.
I plotted the cost per mile for each of MetroMPG's data points, and I had Excel fit a parabola to the result. The optimum speed to drive, considering only time and fuel consumption, is the speed where the cost per mile is the lowest. For me, that would occur around 67mph in the Corolla, and around 62mph in the Blackfly.
However, suppose I was running late for work and I'd be willing to pay $.50 for each minute earlier I arrive, but I still care to conserve gas to the tune of $6/gal. Under these conditions, I'd drive the Blackfly at 68mph, or the Corolla at 78mph.
This is very much back-of-the-napkin. A better optimization problem, like the one that runs in my head whenever I'm behind the wheel, would take in to account that some speeds are not as safe as others (whether too slow or too fast), cars probably wear out faster at higher speeds, and that it's fun to post large MPG numbers. Also, these data only apply to steady-state cruising in economy cars on flat roads with the A/C off, in top gear.
Bottom line: drive faster to save time. Drive slower to save gas. For me, the balance appears to be around 65mph.
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