IMHO a flying car should have a performance improvement while in air.
In L.A., going 40 MPH in the air would be a 1000% improvement over going 4 MPH on the highway.
Seriously though, it's not rocket science. First, you're limited to speed by the speed limit, so it's not a 2.5x difference, it's more like 2x difference. Than, you factor in all of the perks of flight such as direct A to B navigation (Yes, I know that if you need a runway, you can't exactly do A to B navigation, but the idea still holds). The term, "As the crow flies" applies here. Depending on where you are going, being able to fly in a direct line could cut down 20-30% of the travel distance. For instance, I live in Michigan. If I want to go to Wisconsin, I could save a crap ton of time by flying over Lake Michigan. Also, highways don't always go where you want to go. Even a somewhat direct route over 55mph roads with stoplights, stop signs, and traffic would bring your MPH average down significantly.
Having said all that, this company states in TFA that they were designing it for medical missionaries. I'm guessing that in the areas these people are operating, they aren't choosing between I80 or I75 to get to their destination. I'm sure that flying above the trees in Brazil or Africa could save you a ton of time. Also, coming into a primitive village in a flying machine gives you instant "God Cred".