Results from running this on my machine:
$ ruby benchmark.rb
user system total real
ruby_empty_loop 0.010000 0.000000 0.010000 ( 0.011223)
for_loop_no_body 1.550000 0.030000 1.580000 ( 1.576662)
for_loop_with_body 3.030000 0.020000 3.050000 ( 3.058479)