Do you get what Delta & Mark are saying about integers? Integers are whole numbers or "counting numbers" (no decimals or fractions). If you do mathematical operations on integers in C/C++, by default, the result will be returned as an integer, including temporary/intermediate results like (1500/(3900+1500)).
Here's a simple solution... Most of those numbers/calculations are constants, so there's no need to make the Arduino re-calculate every time you run the program...
Please double-check my math, but with your voltage divider and the 5V reference, 14 V should give you an ADC reading of 796, and 1V should give you about 57. You can 'hard code' that constant into your sketch. If my math is correct, all your sketch has to do is divide your count by 57, and you have Volts! (as a truncated integer)
If you want your sketch to treat integers as-if they are floats and get the result as a float, here is a short tutorial about [u]typecasting[/u]. (Of course the actual value in the ADC register is always an integer.)