Fuel usage against the torque curve

Fuel usage against the torque curve

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I have been monitoring MPG in my 1.4L petrol Rover 45 for a few years now using the tank brimming method. I used to drive very sedately on motorways and never go above 65MPH (~3000RPM in 5th) and perhaps I'd be able to get 42MPG on a motorway drive in summer.

Recently, time pressures have meant I've been driving faster. What I've noticed is that my MPG, if anything, has gone up, and at 75MPH, I've been obtaining closer to 44MPG. This is contrary to all advice in the media.
I wondered whether anyone had a torque curve, or efficiency curve for the bog standard 1.4 16V K-series. Is it possible that the engine itself is operating so much more efficiently when sitting nearer 3700RPM that it offsets the increased aero losses? (I have tried googling for the curves, but I don't suppose anyone is usually interested in performance of a 1.4L engine!)

The torque is noticeably better at 3700RPM than 3000, so it seems plausible that the efficiency is better at 3700, but the efficiency/RPM exchange rate would have to be pretty steep to make driving faster use less fuel.

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Fuel usage against the torque curve