So, the contest was held, and the winner...
The new Corolla - with the automatic transmission - came in first (or was last to run out of gas) at 4.9 L/100 km (48 mpg US).
It was followed closely by the Civic (manual transmission) which got 5.3 L/100 km (44.4 mpg US).
The Corolla ran out of fuel after 1017 km, the Civic ran out at 947.
3rd place was the Pontiac Vibe (automatic transmission) @ 854 km (5.9 L/100 km / 39.9 mpg US).
The thirstiest car in the test, a Suzuki SX4, went 637 km before it had guzzled all its fuel.
Full results:
http://www.canadiandriver.com/50litre/main.htm