fill missing data

I have a database of events that are associated with drivers and cars.

These events have the following properties:
date (of start-time)
start-time
end-time
driver-id
car-id
start-time-mileage (STM)
end-time-mileage (ETM)

The list sometimes misses values in the mileage cells.

Could a macro be created to write the proper mileage values?
The rule should be:
if ETM=[empty] then ETM=[STM of next event with the same car-id]
if STM=[empty] then STM=[ETM of previous event with the same car-id]

furthermore: lines that have no startmileage AND no endmileage should be removed.

Link to data is two posts further down...