I have another suggestion for trying to adjust the delay by ear. Going for a perfectly centered image is pretty difficult when you aren't equidistant from the speakers - especially in the tight confines of a car. Arrival time is only one of the factors. You also have amplitude differences and all kinds indirect sound reflecting off your windows.

Instead of going for a centered image try to find the delay where the image smears the least. Find a track with a strong and fairly isolated center image. Human voice is the best because our ears are evolved to hear differences in human voice very well. (I used Joni Mitchell.) Now close your eyes and focus on the voice while moving the delay around. Get a feel for how it changes. Try to find a point where the image seems the most narrow and focused. It's hard to explain but as you move the delay left and right you should be able to hear the image drag and smear in each direction as difference in arrival times becomes greater. With the correct delay the image won't necessarily be centered left/right but it should sound the tightest and most focused. And remember that the perceptual shift will be in the opposite direction as the delay. (i.e. as you turn the knob left and delay the left speaker the sound will shift towards the right because it will be arriving sooner from the right)

I adjusted it by ear first and was very pleased with myself when I got the tape measure and discovered I was spot on.

-Dylan