One half gallon will do an average size car provided you order a color a couple shades darker than your present carpet color to help it make an immediate enhancement.

Tinting guide:  If you want to adjust the color that you order, you can add a little of the various tints listed. For grays, to lighten or give a yellow dinged look, add yellow tint.  To darken grays, add Black tint. You can make the gray  have a green tint, blue tint, or brown tint by adding one of those tints.
For tans and browns, you can make a reddish brown by adding more red to the color or make a greenish brown by adding some of the blue tint. It is good to also have some yellow available when doing this with browns. To make a darker red or burgundy, you can add slowly dark blue, medium blue, black, or burgundy tint.  We advise you to try just a little at a time till you see that you are on the right track to achieve the exact color that you want.  If you take your time, you will be very happy with your work.