I've been looking all over the web for an answer or example of a code that I
can use in a vehicle mileage log.

I have 4 columns:

1. Date
2. Vehicle
3. Start Mileage
4. Ending Mileage

In the vehicle column, I have 3 vehicles (Vehicle 1, Vehicle 2, and Vehicle
3).
When entering an entry into the log, I enter the date, the Vehicle #, then
the start mile (at this point I would like Excel to look back through the
Vehicle column and find the last entry for the particular vehicle # I just
entered and then look at the ending mileage to ensure that the starting
mileage for the new entry is equal to or greater than the ending mileage for
that vehicle number.

Example

ROW DATE VEHICLE START MILEAGE END MILEAGE
1 1/1/2009 Vehicle # 1 10,005 10, 250
2 1/2/2009 Vehicle # 2 100,000 100,025
3 1/2/2009 Vehicle # 3 50,000 50,317
4 1/3/2009 Vehicle #2

Using the example above, when entering row 4, I would like to look back
through the previous entries in the Vehicle Column until finds the last entry
for a particular vehicle (in this case Vehicle #2) then limit my enter into
the Start Mileage Column to a value equal to or greater than the previous
ending mileage (in this case, my value must be equal to or greater than
100,025). If I enter starting mileage that is less than the previous ending
mileage for that vehicle, I would like to recieve an error message

I'm using Excel 2007.

Your assistance is greatly appreciated.