Pour 1/4 to 1/2 inch of salt onto a small, shallow plate. Moisten the rim of a large glass with water and dip into the salt. Fill the glass with ice, and set aside.
Pour the tequila, orange liqueur, and lime juice into a cocktail shaker over ice. Cover, and shake until the outside of the shaker has frosted. Strain into the prepared glass. Slowly pour in grapefruit soda to serve.