[Frank Lee]

One theory is that 1000-1200 ft/mn piston speed is where optimal efficiency is found; I use it when BSFC data is not available.

I've been driving a Mercury Tracer; 1000-1200 piston speed is 42-52 mph. I figure I can't be too far off from optimum going 47 mph- smack dab in the middle.

I don't think there is a single best speed for fe under all conditions. For example when pushing a headwind fe is probably improved by going closer to 42 than 52; conversely when enjoying a tailwind it could be that fe is better going closer to 52 mph, all due to the compromise between higher engine efficiency at higher loads vs the additional hp requirements of higher speed.

It could even be that the car is most efficient MPG-wise at 37 mph, the speed at which the a/t is in top gear and the torque converter can be held in lock-up. Lacking real-time fe instrumentation, I can't say. But it doesn't matter because I wouldn't go 37 mph over that distance anyway in a car.

Incidentally I got 47 mpg on the last tank, which includes using the a/c some. If I drove at the prevailing speeds the car would get mid to high 30s. Only "mods" on this car are grille and brake vent blocks, passenger mirror delete, engine compartment blanket, and one size taller front tires at 45 to 50 psi; due to the a/t there is almost no coasting or engine off.

I've been doing a 120 mile round trip with some regularity lately. Speed limit is 60 so at the limit it would take two hours and at 40 mph it would take three hours. At 47 mph I am getting 10 miles more per gallon vs 55-60 mph saving .7 gallon each round trip, or about $1.85 these days, but it "costs" me 33 minutes which some would say is compensation of $3.36/hour. But I don't think in terms of my free time being worth x dollars/hour (just had to do this math now) plus I don't have a daily commute and do most errand-running on foot thus I spend very little time in cars so I value the higher fe number more than a lower trip time number.