You can specify a umask in transmission’s config file (/etc/transmission-daemon/settings.json). Umask we normally represent in octal, but unfortunately, json does not support that, so we have to convert it to base 10. You can do this in the shell like this:

$ echo $(( 8#022 ))
18

That’s the default, but you probably want 002, which is the same in decimal, so

sudo editor /etc/transmission-daemon/settings.json
# change "umask": 18 to "umask": 2 and save
sudo reload transmission-daemon # tells transmission-daemon to re-read the config 
                                # file. This is important, otherwise, the changes
                                # will be lost

Another thing. If you change the group ownership of the download dir, and add the setgid bit on it, all files created in that directory will have the same group ownership as that directory.

sudo chgrp "$USER" /path/to/transmission/download/dir
sudo chmod g+s /path/to/transmission/download/dir

It will not affect files that already exist. See http://mywiki.wooledge.org/Permissions for more.