diff --git a/test/blackbox-tests/test-cases/pkg/depexts/error-message.t b/test/blackbox-tests/test-cases/pkg/depexts/error-message.t
index 1fba5725782..677c20f69d5 100644
--- a/test/blackbox-tests/test-cases/pkg/depexts/error-message.t
+++ b/test/blackbox-tests/test-cases/pkg/depexts/error-message.t
@@ -8,7 +8,7 @@ Make a library that would fail when building it:
   $ mkdir foo
   $ cat > foo/dune-project <<EOF
   > EOF
-  $ tar -czf foo.tar.gz foo
+  $ tar cf foo.tar foo
   $ rm -rf foo
 
 Make a project that uses the foo library:
@@ -33,8 +33,8 @@ Make dune.lock files
   > (depexts unzip gnupg)
   > (source
   >  (fetch
-  >   (url file://$PWD/foo.tar.gz)
-  >   (checksum md5=$(md5sum foo.tar.gz | cut -f1 -d' '))))
+  >   (url file://$PWD/foo.tar)
+  >   (checksum md5=$(md5sum foo.tar | cut -f1 -d' '))))
   > EOF
 
 Build the project, when it fails building 'foo' package, it shows
diff --git a/test/blackbox-tests/test-cases/pkg/different-dune-in-path.t b/test/blackbox-tests/test-cases/pkg/different-dune-in-path.t
index d57ecc84af7..d05fc46c40e 100644
--- a/test/blackbox-tests/test-cases/pkg/different-dune-in-path.t
+++ b/test/blackbox-tests/test-cases/pkg/different-dune-in-path.t
@@ -12,7 +12,7 @@ Clarify the behavior when the `dune` in PATH is not the one used to start the bu
   >  (allow_empty))
   > EOF
   > cd ..
-  > tar -czf $1.tar.gz tmp
+  > tar cf $1.tar tmp
   > rm -rf tmp
   > }
 
@@ -43,7 +43,7 @@ Make lockfiles for the packages.
   > 
   > (source
   >  (fetch
-  >   (url $PWD/foo.tar.gz)))
+  >   (url $PWD/foo.tar)))
   > 
   > (dev)
   > EOF
@@ -57,7 +57,7 @@ Make lockfiles for the packages.
   > 
   > (source
   >  (fetch
-  >   (url $PWD/bar.tar.gz)))
+  >   (url $PWD/bar.tar)))
   > 
   > (dev)
   > EOF
diff --git a/test/blackbox-tests/test-cases/pkg/e2e.t b/test/blackbox-tests/test-cases/pkg/e2e.t
index 4ad6ab4c00c..ac27e3e8179 100644
--- a/test/blackbox-tests/test-cases/pkg/e2e.t
+++ b/test/blackbox-tests/test-cases/pkg/e2e.t
@@ -19,12 +19,12 @@ Make a library:
   >  (public_name foo))
   > EOF
   $ cd ..
-  $ tar -czf foo.tar.gz foo
+  $ tar cf foo.tar foo
   $ rm -rf foo
 
 Configure our fake curl to serve the tarball
 
-  $ echo foo.tar.gz >> fake-curls
+  $ echo foo.tar >> fake-curls
   $ PORT=1
 
 Make a package for the library:
@@ -46,7 +46,7 @@ Make a package for the library:
   > url {
   >  src: "http://0.0.0.0:$PORT"
   >  checksum: [
-  >   "md5=$(md5sum foo.tar.gz | cut -f1 -d' ')"
+  >   "md5=$(md5sum foo.tar | cut -f1 -d' ')"
   >  ]
   > }
   > EOF
diff --git a/test/blackbox-tests/test-cases/pkg/ocamlformat/helpers.sh b/test/blackbox-tests/test-cases/pkg/ocamlformat/helpers.sh
index be957f2b572..d436055da94 100644
--- a/test/blackbox-tests/test-cases/pkg/ocamlformat/helpers.sh
+++ b/test/blackbox-tests/test-cases/pkg/ocamlformat/helpers.sh
@@ -28,7 +28,7 @@ EOF
 (executable
  (public_name ocamlformat))
 EOF
-  tar -czf ocamlformat-$version.tar.gz ocamlformat
+  tar cf ocamlformat-$version.tar ocamlformat
   rm -rf ocamlformat
 }
 
@@ -55,7 +55,7 @@ build: [
 url {
   src: "http://127.0.0.1:$port"
   checksum: [
-    "md5=$(md5sum ocamlformat-$version.tar.gz | cut -f1 -d' ')"
+    "md5=$(md5sum ocamlformat-$version.tar | cut -f1 -d' ')"
   ]
 }
 EOF
@@ -71,9 +71,9 @@ build: [
   ]
 ]
 url {
-  src: "file://$PWD/ocamlformat-$version.tar.gz"
+  src: "file://$PWD/ocamlformat-$version.tar"
   checksum: [
-    "md5=$(md5sum ocamlformat-$version.tar.gz | cut -f1 -d' ')"
+    "md5=$(md5sum ocamlformat-$version.tar | cut -f1 -d' ')"
   ]
 }
 EOF
@@ -128,7 +128,7 @@ EOF
 (library
  (public_name printer))
 EOF
-  tar -czf printer.$version.tar.gz printer
+  tar cf printer.$version.tar printer
   rm -r printer
 }
 
@@ -145,9 +145,9 @@ build: [
    ]
  ]
  url {
- src: "file://$PWD/printer.$version.tar.gz"
+ src: "file://$PWD/printer.$version.tar"
  checksum: [
-  "md5=$(md5sum printer.$version.tar.gz | cut -f1 -d' ')"
+  "md5=$(md5sum printer.$version.tar | cut -f1 -d' ')"
  ]
 }
 EOF
diff --git a/test/blackbox-tests/test-cases/pkg/ocamlformat/ocamlformat-dev-tool-deps-conflict-project-deps.t b/test/blackbox-tests/test-cases/pkg/ocamlformat/ocamlformat-dev-tool-deps-conflict-project-deps.t
index dbb5ab92dff..645e741baf1 100644
--- a/test/blackbox-tests/test-cases/pkg/ocamlformat/ocamlformat-dev-tool-deps-conflict-project-deps.t
+++ b/test/blackbox-tests/test-cases/pkg/ocamlformat/ocamlformat-dev-tool-deps-conflict-project-deps.t
@@ -25,7 +25,7 @@ Make a fake OCamlFormat which depends on printer lib:
   >  (libraries printer))
   > EOF
   $ cd ..
-  $ tar -czf ocamlformat.tar.gz ocamlformat
+  $ tar cf ocamlformat.tar ocamlformat
   $ rm -rf ocamlformat
 
 Make a printer lib(version 1) that prints "formatted":
@@ -51,9 +51,9 @@ Make a package for the fake OCamlFormat library which depends on printer.1.0:
   >   ]
   > ]
   > url {
-  >  src: "file://$PWD/ocamlformat.tar.gz"
+  >  src: "file://$PWD/ocamlformat.tar"
   >  checksum: [
-  >   "md5=$(md5sum ocamlformat.tar.gz | cut -f1 -d' ')"
+  >   "md5=$(md5sum ocamlformat.tar | cut -f1 -d' ')"
   >  ]
   > }
   > EOF
diff --git a/test/blackbox-tests/test-cases/pkg/ocamlformat/ocamlformat-e2e.t b/test/blackbox-tests/test-cases/pkg/ocamlformat/ocamlformat-e2e.t
index c5e64a995ec..5d75ae6e522 100644
--- a/test/blackbox-tests/test-cases/pkg/ocamlformat/ocamlformat-e2e.t
+++ b/test/blackbox-tests/test-cases/pkg/ocamlformat/ocamlformat-e2e.t
@@ -7,13 +7,13 @@ Exercises end to end, locking and building ocamlformat dev tool.
   $ make_fake_ocamlformat "0.26.3"
 
 Add the tar file for the fake curl to copy it:
-  $ echo ocamlformat-0.26.2.tar.gz > fake-curls
+  $ echo ocamlformat-0.26.2.tar > fake-curls
   $ PORT=1
 
   $ make_ocamlformat_opam_pkg "0.26.2" $PORT
 
 Add the tar file for the fake curl to copy it:
-  $ echo ocamlformat-0.26.3.tar.gz >> fake-curls
+  $ echo ocamlformat-0.26.3.tar >> fake-curls
   $ PORT=2
 
 We consider this version of OCamlFormat as the latest version:
diff --git a/test/blackbox-tests/test-cases/pkg/source-caching.t b/test/blackbox-tests/test-cases/pkg/source-caching.t
index b76119654d9..4992e79e6bb 100644
--- a/test/blackbox-tests/test-cases/pkg/source-caching.t
+++ b/test/blackbox-tests/test-cases/pkg/source-caching.t
@@ -4,10 +4,10 @@ This test demonstrates that fetching package sources should be cached
 
   $ make_lockdir
 
-  $ tarball=source.tar.gz
+  $ tarball=source.tar
   $ sources="sources/"
   $ mkdir $sources; touch $sources/dummy
-  $ tar -czf $tarball $sources
+  $ tar cf $tarball $sources
   $ checksum=$(md5sum $tarball | awk '{ print $1 }')
   $ echo $tarball > fake-curls
   $ port=1