Você não pode usar os aliases de coluna definidos em uma consulta na mesma consulta. Tente algo como o seguinte:
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
