Instead of expensive calculation of precise distance for all of the rows use a bounding box and calculate the exact distance only for rows inside the box.
The simplest possible example is to calculate min/max longitude and latitude that interests you and add it to WHERE clause. This way the distance will be calculated only for a subset of rows.
WHERE
vehicles.gps_lat > min_lat ANDd vehicles.gps_lat < max_lat AND
vehicles.gps_lon > min_lon AND vehicles.gps_lon < max_lon
For more complex solutions see:
- MySQL spatial extensions
- How to use MySQL spatial extensions
- https://stackoverflow.com/a/5237509/342473
