Million of rows

Hi everybody...

I'm new hear, and I will try to explain my problem, and maybe I can
get a help...

I'm writing a software for 3 years, and this software has the position
GPS from vehicles, and other informations.

My problem starts when I had to store all the data about the vehicles,
about 1 or 2 months.

Actually I had a table called DADO_LIDO, that I write all information
and the primary key is DATA (GPS DAY+HOUR) and the VEHICLE IDENTIFY.

Each vehicle trasmit 1 position by 30 seconds, so I have something
like 2000 rows per vehicle/day. I already has 2 clients one with 4000
vehicles, and the other with 500 vehicles.

My application was made in delphi using ZEOS that's permit me testing
in mysql and postgres.

I allready has the two databases.

But now the problem starts when I has to select data from this
vehicles about the history ( I store only 2 months ) something like 40
or 50 millions of data about 500 vehicles.

Using the keys VEHICLE_ID and GPS_TIME, the perfomance is very low...

I need some ideas for a better perfomance in this table

using selects by
PERIOD / VEHICLE
PERIOD / VEHICLES
PERIOD / VEHICLE / ( a bit test in 3 integer columns using logical operators )

Thanks for any help

Vinicius Marques De Bernardi