Get data from..yesterday?

Hi, i've a table where i put every day the start and the end of Kilometers of any veicle.

for example
DAY----Vecile----KmStart----Km End

04------1----------10-----------20

04------2---------200----------300

05------1----------20-----------30

05------2---------300----------451

06------2---------451----------467

ecc...
The end of day 04 for veicle 1 is 20 Km like the start of day 05 for veicle 1
I want to report this '20' automaticaly in the add page

I've do this in add page: before display

i want to put the '20' in the field KmInizio automaticaly.

But it don't works... off course, The search should seek at the maximum

value, and not in the yesterday value in that vehicle. <img src='https://asprunner.com/forums/file.php?topicimage=1&fieldname=question&id=13698&image=1&table=forumtopics' class='bbc_emoticon' alt=':unsure:' />
global $conn;

$id = $values["IdTarga"];

$str = "select KmFine from tabelladischetti where Id='".$id."'";

$rs = db_query($str,$conn);

$data = db_fetch_array($rs);

$KmFine = $data["KmFine"];

$xt"KmInizio";