There are a lot of ways to assign value to a car. Most evaluations are based on the age of the car and the number of miles it has been driven. If your car has been driven considerably fewer miles than expected for the age of your car, you may be able to argue that it is worth more than most comparably aged cars.
Also, if you have refurbished the car in some way, it may have more value than similarly aged cars. Your car insurance attorney can help you dispute an evaluation that you think undervalues your vehicle.