Something

SELECT DISTINCT ON (maxspeed, x.assetid) x.assetid,
      x(transform(x.position, 99999)) AS xpoint,
      y(transform(x.position, 99999)) AS ypoint,
      ts, maxspeed, fleet, tagid, heading, poweron
  FROM
  ( 
    SELECT ap.*, maxspeed, apidx FROM(
  
      SELECT assetid
       , max((speed::NUMERIC * (10^22)::NUMERIC) + asset_positions.id)::NUMERIC as apidx
       , max(speed) as maxspeed
      FROM asset_positions
      WHERE Distance(asset_positions.position, transform( GeomFromEWKT('SRID=99999;POLYGON((-9231299.906000735 4839195.409207389 0,-9231414.561543163 4828879.028734494 0,-9250256.28901546 4828079.607960547 0,-9250753.129699312 4840489.72123508 0,-9231299.906000735 4839195.409207389 0))'), 4326 )) = 0
      AND ts between '2007-10-05T00:00:00-07:00'::TIMESTAMP AND '2007-10-05T23:59:00-07:00'::TIMESTAMP
      AND assetid IS NOT NULL
      AND speed::NUMERIC > 15277.777778
      GROUP BY assetid   
    ) AS innertable
    INNER JOIN asset_positions ap
      ON ap.id = (apidx - (maxspeed::numeric*(10^22)::NUMERIC))::BIGINT
    WHERE ap.id = (apidx - (maxspeed::numeric*(10^22)::NUMERIC))::BIGINT
  ) x
  INNER JOIN asset ON asset.id =x.assetid
  WHERE asset.id=x.assetid