I'm not a Mt. Dew drinker, so I'm not sure why made this. It's not a perfect match, but it's a start and I'd love to have some feedback here. Probably for a better match you'll want to skip the peels, they add some citrusy bitterness that I just can't live without, but may be nasty for other people.
2 cups sugar, inverted in 1 cup water
2 oranges (about 1/2 cup)
2 lemons (about 1/2 cup)
2 limes (about 1/4 cup)
1 cup water
Carbonated water up to 1 gallon.
Combine sugar, 1 cup water, and cream of tartar. Bring to a boil and simmer 20 minutes. Allow to cool. Bring the other 1 cup of water to a boil separately. Add the citrus peels, remove from heat and cover. Allow to steep until it has cooled to room temperature. Strain and add to sugar solution, along with the strained juices Ensure it is well blended. Add to carbonated water and mix gently until dissolved.