diff --git a/lib/git_manager.rb b/lib/git_manager.rb
index 72149e0..d084bc5 100644
--- a/lib/git_manager.rb
+++ b/lib/git_manager.rb
@@ -1,5 +1,7 @@
+require 'fileutils'
 require 'logging'
 require 'running'
+require 'shellwords'
 
 # Lightweight wrapper over Git, shells out everything.
 class GitManager
@@ -13,7 +15,7 @@ def initialize(basedir)
   end
 
   def remote_rails_url
-    'https://github.com/rails/rails.git'
+    "#{github_rails_url}.git"
   end
 
   def update_main
@@ -38,8 +40,17 @@ def update_main
 
   def checkout(tag)
     Dir.chdir(basedir) do
-      log "checking out tag #{tag}"
-      log_and_system "git -c advice.detachedHead=false clone -q --depth 1 --single-branch --branch #{tag} #{remote_rails_url} #{tag}"
+      log "fetching archive for tag #{tag}"
+      FileUtils.rm_rf(tag)
+      FileUtils.mkdir(tag)
+
+      archive_url = remote_archive_url(tag)
+      log "downloading #{archive_url}"
+
+      Dir.chdir(tag) do
+        command = "curl -sSL #{Shellwords.shellescape(archive_url)} | tar -xzf - --strip-components=1"
+        log_and_system 'sh', '-c', command
+      end
     end
   end
 
@@ -58,4 +69,14 @@ def sha1
       `git rev-parse HEAD`.chomp
     end
   end
+
+  private
+
+  def remote_archive_url(tag)
+    "#{github_rails_url}/archive/refs/tags/#{tag}.tar.gz"
+  end
+
+  def github_rails_url
+    'https://github.com/rails/rails'
+  end
 end
diff --git a/test/git_manager_test.rb b/test/git_manager_test.rb
index 1861620..6305b2a 100644
--- a/test/git_manager_test.rb
+++ b/test/git_manager_test.rb
@@ -1,6 +1,7 @@
 require_relative 'test_helper'
 
 require 'git_manager'
+require 'shellwords'
 
 class GitManagerTest < Minitest::Test
   def create_repository
@@ -27,10 +28,16 @@ def test_checkout
         system 'echo t2 > README'
         system 'git commit -aqm "test"'
         system 'git tag t2'
+
+        %w(t1 t2).each do |tag|
+          archive_path = File.expand_path("../#{tag}.tar.gz", Dir.pwd)
+          system 'sh', '-c', "git archive --format=tar --prefix=rails-#{tag}/ #{tag} | gzip > #{Shellwords.escape(archive_path)}"
+        end
       end
 
       gm = GitManager.new('basedir')
-      gm.stub('remote_rails_url', "file://#{Dir.pwd}/basedir/main") do
+      archive_dir = File.expand_path('basedir')
+      gm.stub(:remote_archive_url, ->(requested_tag) { "file://#{File.expand_path("#{requested_tag}.tar.gz", archive_dir)}" }) do
         gm.checkout('t1')
       end