NASA in 2011 sponsored a Green Flight Challenge, promoting fuel efficient aircraft development. Synergy, a diesel powered propeller aircraft entered in the competition reached speeds of 100-450 miles per hour, seating five people and got 40 miles per gallon burning diesel fuel.
When I commuted between Oxnard Airport OXR and Monterey Airport MRY in the late 1960s in a two-place Cessna 150 with 100 Hp burning aviation fuel I got about 21 miles per gallon, for comparison.
Modern jetliners use 36,000 gallons of jet fuel on a ten hour flight. Smaller jets do better, but aren't all that efficient. Synergy was ten times better in fuel efficiency than small jets. See Wikipedia for more information on Synergy and the Deltahawk diesel engine.
Synergy is by far not the first diesel-powered aircraft. The Stinson SM1-B DETROITER production aircraft holds the distinction of being the World's first diesel-powered airplane. See my photos here of N7654X, a successful airworthy 1928 Stinson SM1-B DETROITER production aircraft with diesel power taken in Greg Herrick's Golden Wings Museum at Anoka County Airport ANE where, incidentally, I learned to fly well before the museum's establishment.