I've just seen your reply. Thanks a lot!
I'll measure the distances and post them here. Since I have separated woofers and tweeters (woofers in doors and tweeters in sail panels, behind the side mirrors), the distances are different. From what I read, human hear is more sensitive to time differences in the lower frequencies and to volume level in the higher frequencies, so the distance between the woofers seems more relevant in this case.
Anyway, my previous experience with time delay, two cars ago using an Alpine headunit, tells me that the differences are only a starting point and that ultimately, the delay is set by hearing.
I think most people that hear music in their cars do not realize what a difference time delay can make to imaging and soundstage, so this would really be a great feature for future versions of rockbox.
A good additional feature would be to apply the delay to the right channel, just for those Brits who drive on the wrong side of the car
(not my case, anyway)