For an authentic and refreshing tinto de verano, the best dry variety to look for at the supermarket is a young Spanish red, particularly Tempranillo (specifically labeled Joven) or Garnacha (Grenache).
The reason these work so well comes down to tannin and oak levels. Tinto de verano is served ice-cold and mixed with a carbonated mixer such as lemon soda or sparkling water. When a heavily oaked, high-tannin wine like a Cabernet Sauvignon or an aged Reserva is chilled, the tannins can turn unpleasantly bitter and astringent on the palate. In contrast, young Tempranillo and Garnacha are naturally fruit-forward, medium-bodied, and relatively low in harsh tannins, allowing the bright red fruit notes to blend smoothly with the sweetness and acidity of your mixer.
When browsing the aisles tonight, you do not need an expensive bottle. In fact, standard entry-level Spanish table reds in the affordable price range are ideal. If your local shop does not have a Spanish section, a basic, fruit-forward Merlot or an un-oaked Syrah makes a very capable substitute.
To mix it, simply combine equal parts chilled red wine and lemon soda over plenty of ice, then garnish with a fresh slice of lemon or orange.