diff --git a/project.py b/project.py
index 61fddac..00e41ad 100644
--- a/project.py
+++ b/project.py
@@ -529,7 +529,8 @@
                upstream=None,
                parent=None,
                is_derived=False,
-               dest_branch=None):
+               dest_branch=None,
+               optimized_fetch=False):
     """Init a Project object.
 
     Args:
@@ -551,6 +552,8 @@
       is_derived: False if the project was explicitly defined in the manifest;
                   True if the project is a discovered submodule.
       dest_branch: The branch to which to push changes for review by default.
+      optimized_fetch: If True, when a project is set to a sha1 revision, only
+                       fetch from the remote if the sha1 is not present locally.
     """
     self.manifest = manifest
     self.name = name
@@ -579,6 +582,7 @@
     self.upstream = upstream
     self.parent = parent
     self.is_derived = is_derived
+    self.optimized_fetch = optimized_fetch
     self.subprojects = []
 
     self.snapshots = {}
@@ -1060,7 +1064,8 @@
       current_branch_only=False,
       clone_bundle=True,
       no_tags=False,
-      archive=False):
+      archive=False,
+      optimized_fetch=False):
     """Perform only the network IO portion of the sync process.
        Local working directory/branch state is not affected.
     """
@@ -1129,8 +1134,9 @@
       elif self.manifest.default.sync_c:
         current_branch_only = True
 
-    has_sha1 = ID_RE.match(self.revisionExpr) and self._CheckForSha1()
-    if (not has_sha1  #Need to fetch since we don't already have this revision
+    need_to_fetch = not (optimized_fetch and \
+      (ID_RE.match(self.revisionExpr) and self._CheckForSha1()))
+    if (need_to_fetch
         and not self._RemoteFetch(initial=is_new, quiet=quiet, alt_dir=alt_dir,
                                   current_branch_only=current_branch_only,
                                   no_tags=no_tags)):
@@ -1836,6 +1842,8 @@
       cmd.append('--quiet')
     if not self.worktree:
       cmd.append('--update-head-ok')
+    if self.manifest.IsMirror:
+      cmd.append('--prune')
     cmd.append(name)
 
     # If using depth then we should not get all the tags since they may
diff --git a/subcmds/sync.py b/subcmds/sync.py
index 2bdab3a..b4546c1 100644
--- a/subcmds/sync.py
+++ b/subcmds/sync.py
@@ -131,6 +131,10 @@
 The -c/--current-branch option can be used to only fetch objects that
 are on the branch specified by a project's revision.
 
+The --optimized-fetch option can be used to only fetch projects that
+are fixed to a sha1 revision if the sha1 revision does not already
+exist locally.
+
 SSH Connections
 ---------------
 
@@ -206,6 +210,9 @@
     p.add_option('--no-tags',
                  dest='no_tags', action='store_true',
                  help="don't fetch tags")
+    p.add_option('--optimized-fetch',
+                 dest='optimized_fetch', action='store_true',
+                 help='only fetch projects fixed to sha1 if revision does not exist locally')
     if show_smart:
       p.add_option('-s', '--smart-sync',
                    dest='smart_sync', action='store_true',
@@ -275,7 +282,8 @@
           quiet=opt.quiet,
           current_branch_only=opt.current_branch_only,
           clone_bundle=not opt.no_clone_bundle,
-          no_tags=opt.no_tags, archive=self.manifest.IsArchive)
+          no_tags=opt.no_tags, archive=self.manifest.IsArchive,
+          optimized_fetch=opt.optimized_fetch)
         self._fetch_times.Set(project, time.time() - start)
 
         # Lock around all the rest of the code, since printing, updating a set
@@ -615,7 +623,8 @@
     if not opt.local_only:
       mp.Sync_NetworkHalf(quiet=opt.quiet,
                           current_branch_only=opt.current_branch_only,
-                          no_tags=opt.no_tags)
+                          no_tags=opt.no_tags,
+                          optimized_fetch=opt.optimized_fetch)
 
     if mp.HasChanges:
       syncbuf = SyncBuffer(mp.config)