Sammenkædning af rækker fra en alias beregnet kolonne

Dette virker for mig (det ville være nemmere, hvis du gav nogle data at teste med):

SELECT
  C.CarName,
  SUBSTRING((
             SELECT
              ', ' + CAST(CarMileage / TotalGasTrips AS VARCHAR(MAX))
             FROM
              Mileage
             WHERE
              Mileage.CarName = C.CarName
             GROUP BY
              dbo.Mileage.CarMileage,
              dbo.Mileage.TotalGasTrips
            FOR
             XML PATH('')
            ), 2, 1000) AS MPGString
FROM
  Car C
  JOIN dbo.Mileage M ON C.CarName = M.CarName
GROUP BY
  C.CarName