Fórum AJUDEM #458448

resolvi ficou assim

SELECT
VI.ID_MAN_ITENS, VI.ID_MANUT, VI.ID_PROD, VI.DESCRICAO,
VI.UNID, VI.QUANTIDADE, VI.UNITARIO, VI.TOTAL, VM.HODOMETRO,
VM.ID_MOTORISTA, VM.ID_FORN, VM.ID_VEIC,

(
(VM.HODOMETRO -
(COALESCE(

( SELECT FIRST 1 VMI.HODOMETRO
FROM VEIC_MAN_ITENS V
LEFT JOIN VEIC_MANUT VMI ON VMI.ID_MANUT = V.ID_MANUT
WHERE (V.ID_PROD = VI.ID_PROD)
AND (V.ID_MAN_ITENS<VI.ID_MAN_ITENS)
ORDER BY V.ID_MAN_ITENS DESC
),

( SELECT VEI.HODOMETRO
FROM VEICULOS VEI
WHERE (VEI.ID_VEIC=VM.ID_VEIC)
)
)
)
)/VI.QUANTIDADE
) AS MEDIA

FROM
VEIC_MAN_ITENS VI

LEFT JOIN VEIC_MANUT VM ON VM.ID_MANUT = VI.ID_MANUT


WHERE (NOT VI.ID_MAN_ITENS IS NULL)

Finalizando!