Force Apache HTTP compression event if no Accept-Encoding header was sent

Addition to Martin Fjordvalds answer:

Apache uses chunked encoding only if the compressed file size is larger than the DeflateBufferSize. Increasing this buffer size will therefore prevent the server using chunked encoding also for larger files, causing the Content-Length to be sent even for zipped data.

More Information is available here: http://httpd.apache.org/docs/2.2/mod/mod_deflate.html#deflatebuffersize

If you are serving a .gz file, perhaps you are looking to add an association between the file extension and the desired mime-type with mod_mime. The following can be added to the VirtualHost configuration.

AddEncoding x-gzip .gz

Alternatively, a more complete example for serving pre-compressed content involves not just adding the encoding-type but also disabling mod_deflate and resetting the content-type. This example uses mod_header to add the encoding-type.

<IfModule mod_headers.c>
    # Serve gzip compressed CSS files if they exist 
    # and the client accepts gzip.
    RewriteCond "%{HTTP:Accept-encoding}" "gzip"
    RewriteCond "%{DOCUMENT_ROOT}%{REQUEST_FILENAME}\.gz" -s
    RewriteRule "^(.*)\.css" "$1\.css\.gz" [QSA]

    # Serve gzip compressed JS files if they exist 
    # and the client accepts gzip.
    RewriteCond "%{HTTP:Accept-encoding}" "gzip"
    RewriteCond "%{DOCUMENT_ROOT}%{REQUEST_FILENAME}\.gz" -s
    RewriteRule "^(.*)\.js" "$1\.js\.gz" [QSA]


    # Serve correct content types, and prevent mod_deflate double gzip.
    RewriteRule "\.css\.gz$" "-" [T=text/css,E=no-gzip:1]
    RewriteRule "\.js\.gz$" "-" [T=text/javascript,E=no-gzip:1]


    <FilesMatch "(\.js\.gz|\.css\.gz)$">
      # Serve correct encoding type.
      Header append Content-Encoding gzip

      # Force proxies to cache gzipped & 
      # non-gzipped css/js files separately.
      Header append Vary Accept-Encoding
    </FilesMatch>
</IfModule>

(Note the above adds DOCUMENT_ROOT to the RewriteCond file path from the referenced sample since I believe it is needed when used in the VirtualHost context from 2.2+.)