Some info pages missing

On Debian and its derivatives like Ubuntu, the info pages are not installed unless you install the corresponding package-doc package for a given package.

So in your case:

apt-get install tar-doc

A notable exception (though that may only apply to Debian and not Ubuntu) is bash-doc. The textinfo bash documentation is not considered as free software by Debian as you're not free to modify it (you have to notify the bash maintainers if you want to distribute a modified version of it which is against the Debian policy).

There's a similar case for texinfo-doc though in that case there is a texinfo-doc-nonfree package.

On my Ubuntu 12.04 system when I run the command info tar I actually get the man page for tar. You can see what file info is using with the -w switch.

$ info -w tar
*manpages*

$ info -w ls
/usr/share/info/coreutils.info.gz

If you search the repos you'll find this:

$ apt-cache search info|grep tar
...
pax - Portable Archive Interchange (cpio, pax, tar)
tar-doc - documentation for the tar package
...

Install the tar-doc package for the info.

There might be an easier way to do this (not that up on apt..) but you can find all the missing info doc files with this command:

$ apt-cache search info | grep -- '-doc '