You can use this project to compare a clean Rails 2 application vs. Rails 3. See the following blog post regarding this simple_benchmark: http://blog.tstmedia.com/news_article/show/86942  

An empty User model, a migration for adding a users table with a user_name column, and a migration for populating the users table with 10,000 users is all that was added to each rails app. These rails apps are setup to use mysql. The rails 2 app uses the mysql driver and the rails 3 app uses the mysql2 driver. To run the benchmark yourself, create the databases rails3_simple_benchmark_production and rails2_simple_benchmark_production in mysql, modify the database.yml file if needed (username, password, etc.), run migrations, and then execute the following:

Rails2: 
env RAILS_ENV=production rails2/script/performance/benchmarker "10000.times { |i| u = User.find_by_id(i); u.user_name if u }"

Rails3:
env RAILS_ENV=production rails3/script/rails benchmarker "10000.times { |i| u = User.where(:id => i).first; u.user_name if u }"