I have already taken photographs of some NYC Checker cabs, while traveling in NYC last summer. The only real issues, is that there is snow everywhere, and to shoot for the warmer months of the year, would be slightly difficult at this time, which is why I placed a 30 Days period on this.
To do this photo shoot, it is fairly simple. You go to the actual cab company and notify them that you are interested in photographing their vehicles in NYC. Sometimes you get lucky by them offering a cab and driver for a short period of time, so you can drive it out to different locations (Like the harbor, or Central Park). The key is to figure out what feel you want for the Calendar. - Do you prefer no individuals in the photographs, Is it acceptable to have the cab drivers in the shot, Do you want more industrial/edgy look to the calendar... These questions would obviously be more clear once the process begins.
After each trip to the city, I will look through the photographs and forward you the ones I deem as acceptable - With this approach it should only take a handful of trips to achieve EXACTLY what you are looking for.