script.rb

require "benchmark"

TESTS = 1_000_000
array = [1,2,3]
Benchmark.bmbm do |results|
  results.report " for" do
    TESTS.times do
      for a in array
        "foo"
      end
    end
  end
  results.report "each" do
    TESTS.times do
      array.each do |a|
        "foo"
      end
    end
  end
end

The results

Rehearsal ----------------------------------------
 for   2.490000   0.030000   2.520000 (  2.947099)
each   2.630000   0.030000   2.660000 (  3.169486)
------------------------------- total: 5.180000sec

           user     system      total        real
 for   2.510000   0.030000   2.540000 (  3.117419)
each   2.630000   0.040000   2.670000 (  3.330009)