SQL dividiendo 2 valores de 2 consultas

Básicamente, trate esas dos consultas como subconsultas, como se muestra a continuación.

select x.number / y.number 
from
(
  SELECT COUNT(cvu.[ID]) as number, 'Exp' AS [Exp]     
  FROM [dbo].[tblClientVehicleUnit] cvu     
  WHERE ExpirationDate < GetDate()     
  AND cvu.Id = '4C1' 
) x
join 
(
  SELECT COUNT(cvu.[ID]) as number, 'NonExp' AS [Exp]     
  FROM [dbo].[tblClientVehicleUnit] cvu     
  WHERE ExpirationDate > GetDate()     
  AND cvu.Id = '4C1'
) y on 1=1

Si quisiera ir más allá, podría tener el cvu.id como parte de la selección y modificar la unión para poder hacerlo en todos los cvu.id

select x.id, x.number / y.number 
from
(
SELECT cvu.id, COUNT(cvu.[ID]) as number, 'Exp' AS [Exp]     
FROM [dbo].[tblClientVehicleUnit] cvu     
WHERE ExpirationDate < GetDate()     
group by cvu.Id 
) x
join 
(
SELECT cvu.id, COUNT(cvu.[ID]) as number, 'NonExp' AS [Exp]     
FROM [dbo].[tblClientVehicleUnit] cvu     
WHERE ExpirationDate > GetDate()     
group by cvu.Id 
)y on x.id = y.id