Data loading query

[#1]

I have some data which looks like

How to I write a load script such that each sequential entry for car B, for example gets an incremental number so instead of above the data would look like

etc. Any ideas much appreciated

[#2]

My original data has date and time as one field, I could split this out but don't want to, how could I make the numbering restart for second day so  might bet number 1-6 for day 1 and number 1-4 for day 2 for the same car?

[sunny_talwar]

May be like this:

Autonumber(RowNo(), CarNumber&Floor(DateTime)) as number,