Excellent demonstrations of what is happening, Vikki and Smitty!
A couple of things at work here. The wider the throttle opening, the more efficient the engine is. Why? Because at low throttle, the amount of air coming in is also low and the effective (or dynamic) compression ratio is low. At high throttle openings, the cylinders are getting filled properly and the engine is running at a high dynamic compression ratio. The higher the compression ratio, the more efficient the combustion process.
At the engine's torque peak and full throttle, the cylinders are getting their maximum amount of air per cycle, and the engine will be operating at its maximum compression ratio. The power made with a given amount of fuel will be at its peak as well (BSFC). Based on this, some people advise that the car should be geared to cruise with the engine at its torque peak. However, this usually is too high a gear as the engine won't require much throttle opening, and the throttle opening has a much greater effect on fuel mileage than the torque peak because of the difference in dynamic compression. It's better to have the engine running at the lowest rpm that it will run cleanly than to worry about the torque peak.
The running clean also is important, more so for carbs than EFI. If you are running the carb on the idle and transition circuits (light throttle), the metering is usually richer and not as accurate as the main circuit. The atomization of the fuel as it comes out of the boosters is not quite as good either, so more throttle opening will be more efficient. However, getting into the power circuit will again cause the carb to run richer and decrease fuel mileage. If you know where your power circuit is calibrated to come in, you can use a vacuum gauge and run the car a couple inches of vacuum above that for the best mileage. That will be the "sweet spot" for your carb.
A Pontiac example: typical "hot" 400, ported #48 heads, RA IV cam and headers. I ran this with a 2.56 gear and a TH400 (2400 rpm at 70 mph) and got about 19 mpg (US), switched to a 3.23 gear and a 4-speed (3000 rpm - closer to the torque peak) and got 16 mpg. Ran at about 10-12" vacuum on level road.
Vikki, the best mileage will be had when you use full throttle and keep the revs close to the torque peak - as long as you don't have to brake for slower traffic - so pretty close to what you're doing! The worst mileage is light throttle and high revs.
