First, welcome to 8052.com. Regarding your question, yes it is possible to make a car-computer with an 80c51. And (just in case you aren't confused enough yet) no it isn't. Now let me explain.

The term "car-computer" is extremely broad and poorly defined. First, before you do anything else, define specifically what you want your car-computer to do. Do you want it to control the fuel flow and/or injectors? Or do you want it to datalog trip statistics via a GPS receiver? And also, just as a bit of free advice, avoid the temptation to try and make your project do too much. Doing one thing well is far more impressive than doing five things half-well (and yes, I know that "well" isn't the traditional ending to that colloquialism).