The claims seem perfectly plausible to me. Manual gear box alone will contribute to a 3-5 MPG improvement. Low speed driving, in top gear are ideal conditions for fuel economy maximizing.

I did a fuel economy "race" with Hyundai once with the Hybrid Sonata, tanks were sent out half full to reduce weight, driven 100 miles and using the car's onboard trip computer to calculate MPG (no argument this method isn't considered "accurate" by most). Still, my winning run was 62.4 MPG AND I also average the highest average speed of the group of 54 MPH. How? I changed the prescribed route, avoiding the route with many stop lights, and taking the freeway, sticking to the right lane and treating the accelerator as though I had an egg under it.

Fuel economy is as much about the foot on the pedal as it is anything else...and is the biggest "variable" there is when discussing MPG experiences.

Further I don't see the need to so vehemently challenge the voracity of a claim that isn't preposterous.

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1983 W123 300TD US spec Turbo engine, with Euro bumpers and manual climate control, and manual transmission.