Referencia no admitida en la función de grupo

No puede usar los alias de columna que definió en una consulta en la misma consulta. Prueba algo como lo siguiente:

SELECT   *,
         TRIP1 / FUEL1 AS ECON1,
         TRIP31 / FUEL31 AS ECON31,
         OTHER COLUMNS....
FROM
(
    SELECT    v.V_VEHICLEID, 
              v.V_LICENSENO,
              ROUND((MAX(IF(DAY(WP_DATETIME) = 1, WP_ODOMETER, NULL)) - MIN(IF(DAY(WP_DATETIME) =     1,     WP_ODOMETER, NULL))) / 1000) AS TRIP1,
              ROUND(MAX(IF(DAY(WP_DATETIME) = 1, WP_FUELREAD, NULL)) - MIN(IF(DAY(WP_DATETIME) = 1,     WP_FUELREAD, NULL)),2) AS FUEL1,
              ROUND((MAX(IF(DAY(WP_DATETIME) = 31, WP_ODOMETER, NULL)) - MIN(IF(DAY(WP_DATETIME) = 31, WP_ODOMETER, NULL))) / 1000) AS TRIP31,
              ROUND(MAX(IF(DAY(WP_DATETIME) = 31, WP_FUELREAD, NULL)) - MIN(IF(DAY(WP_DATETIME) = 31, WP_FUELREAD, NULL)),2) AS FUEL31,
              OTHER COLUMNS...
    FROM      VEHICLES v
    JOIN      WAYPOINTS wp
    ON        wp.WP_VEHICLEID = v.V_VEHICLEID
    WHERE     MONTH(wp.WP_DATETIME) = '6' 
    AND       v.V_USER = 'tc'
    GROUP BY  wp.WP_VEHICLEID
) t