Michael In Germany, "Spezi" is a popular designated driver drink, but it's nowhere to be found on supermarked shelves within the U.S., and neither are similar German products like "Mezzo-Mix". By accident, I recently found a combination of American ingredients that create an astonishingly great tasting interpretation: Ice cubes, a slice of organic lemon including skin, 4/5 of Coke Zero and 1/5 of American regular Fanta.
Fanta in the U.S. is quite different from the Fanta sold in Europe. It contains loads of unreal bright orange dye, and tastes a lot sweeter due to the added corn syrup which is heavily subsidized by U.S. tax payers. Careful, it's so sweet, it wouldn't surprise me to see unsuspecting Europeans go into a serious episode of hyperglycaemia! Coke Zero, on the other hand, contains a somewhat controversial artificial sweetener, similar to Diet Coke, but at least Coke Zero tastes almost like Coke and not like cardboard.
It might sound unlikely that he mixture above actually closely resembles the taste of "Spezi", but believe me, I've got fine-tuned taste buds for this kind of thing. I demand that German restaurants like the "Suppenküche" in San Francisco quickly put this fabulous drink on their menu!