Gassing Downhill, Coasting uphill. A fuel efficiency question.

[Urwumpe]

Particularly if it's a modern highly strung low cc engine.

Especially those. An old large displacement, long stroke Diesel engine does not complain as long as you have still rotations per minute.

 

[Cairan]

On our 2007 Mazda 3, after 260 000 km and still going strong, with a diagnostic computer reader, I've figured the best numbers for fuel efficiency and practical speeds:

Cruise 3200 RPM, in proper gear to maintain at or below target RPM
Downhill 3800 RPM in proper gear to maintain at or above target RPM
Uphill one gear lower than the one engaged before going uphill