Insane mileage under load

I drove about 100 miles to a meet and my mileage seemed about normal, the fuel gauge dropped the normal amount of bars. On the return trip I was hauling a very nice aluminum trailer. (Can you tell I'm a bit of a trailer snob? I don't like hauling anything other than a mostly aluminum trailer or using an aluminum cargo carrier)

On my return trip I was pushing my car a bit over normal. I kept it at 65-67mph and to my astonishment two things were happening, 1) My Mirage was pulling this new trailer like it wasn't even there. I was incredibly impressed. The only time I knew I had a trailer is when there were substantial bumps in the road.

2) My car didn't drop off the half full mark until I hit 320kms. Normally with conservative driving my gauge drops one bar below the half mark somewhere around 275kms. I DID fuel up at a different station today as my normal station ran out of gas! Where I gassed up today has in the past generally delivered poorer mileage.

The only thing that comes to mind is perhaps my engine was kept in the optimum rpm range for the additional load on the engine. I would think the engineers at Mits would have different fuel metering profiles based on the load the engine is facing. I'm sure everything is tuned to the optimum, ignition timing BTDC, valve opening/closing lift and duration and fuel injected. I'm on the way to getting 400 miles from a tank of gas, something I've never done before.