I agree with Fandus as far as the state of the room you're mixing. Dead rooms tend to lead to boosting to compensate, but when listened outside of that environment, all those nuances you thought you nailed become problems. I quick way to check your levels is to balance everything in mono ... it's good way to hear if something is overbearing your mix. Also, even though it may sound fine in those Rokit 6's (I mix with RP-8s), depending on where they're located, what surface they're on, if there is any noise suppression going on (I have some Aurelex foam riser-like pads under my monitors), the angle and spacing of the monitors, the state of the room, etc etc etc ... there's TONS of factors that can be at play, but another good way is to check the mix using different headphones. I personally check with iPod earbuds, Skull Candiy earbuds, some Bose headphones just to guage how it would sound to the everyday person (still do the majority of my mixing on some Beryerdynamics DT-880 Pros).
Checking the car for reference is great though it can cloud a mix based on your speaker set-up/location, size of your subs, and if you have any onboard dash EQ applied, etc etc. I do find a car can add some additional low/lower-mid just based off it's acoustics. I would also check the mix in a car that has a stock stereo system. At the end of the day, not everyone is an audiophile and has top notch gear, so check everything! I would listen to another mix with the settings you normally listen with in your car to see how it compares to your mix. Take a look at a frequency analyzer to see problem areas and compare. The best thing to do is to honestly use your ears