The uuencode is what makes the file an attachment, so if you mess with it after encoding it, it will no longer be taken to be an attachment.
Obviously uuencode doesn't understand Excel sheets, so you can compress the data *before* the uuencode.
gzip -9 SAC_Tire.xls
uuencode SAC_Tire.xls.gz SAC_Tire.xls.gz | mail -s "SAC Weekly Tire Report" Mailid@yahoo.com
that looks closer to me. can't test it here though.
Read the link chrism01 gave, that's useful.