I don't know about the logistics of fitting a mug with a sippy cup top in my car's cup holder, but I do know about the logistics of being able to walk down the street drinking from my favorite ninja mug without sloshing and scalding a red beard onto my face with the best coffee drink on earth my mama makes me before work every morning (it's coffee mixed with half a pouch of powdered hot chocolate. The kind that also contains freeze-dried marshmallows). Those logistics require a disposable cup with a lid (cancerous to Mother Nature), a travel mug (not my favorite ninja mug), or an Ecotop.
Ecotops fit over any container with a 3" diameter mouth. In addition to coffee mugs, this can include many cold beverage glasses as well. Like, say, my dual beer glass, which once I leave my godforsaken state of archaic liquor laws for the boozy utopia that is New Orleans next week, I may need in easy-access, traveling form. The flexible, press-on Ecotops have a spouted, sealed designed both to facilitate sipping while moving and prevent spillage in unstable settings or while in the hands of a spastic child. That means the little kiddos can drink out of big kiddo glasses fitted with an Ecotop too.
Available in 7 different colors, Ecotops are microwaveable and dishwasher safe.