You are right, multiplying with 0.01 is even fast, see the benchmarks:

require 'benchmark'

times = 1000000
float = 9.234234765765765764
Benchmark.bm do |r|
	r.report('SPf  :') {
	  times.times do
	    f1 = sprintf('%.2f' ,float).to_f
    end
  }
	r.report('*100 :') {
	  times.times do
	    f2 = (100 * float).round/100.0
    end
  }
	r.report('*.01 :') {
	  times.times do
	    f3 = (100 * float).round*0.01
    end
  }
end

        user     system      total        real
SPf  :  5.590000   0.050000   5.640000 (  5.720373)
*100 :  3.760000   0.040000   3.800000 (  3.829771)
*.01 :  1.770000   0.010000   1.780000 (  1.811917)

Thomas