CRJ200 First Officer reports GEAR DISAGREE warning after gear retraction and subsequent return after safe down indication is achieved. Reporter does not accurately compute landing weight however and lands overweight without knowing; using incorrect reference speeds.

Narrative

Shortly after gear retraction upon takeoff we received a 'GEAR DISAGREE' warning message. Ran the QRH procedure; had three green 'DN' gear indications; and returned to the departure airport. We used ACARS to compute the landing data which put us on the 47;000 LB card. After further investigation after safely back at the gate we realized the landing weight was based on a burn-off to our destination; and we had in fact inadvertently landed roughly 1000 LBS overweight. After contacting Maintenance back at the gate; they asked whether an overweight landing inspection would be required. We initially replied that that would be unnecessary. At that point we took a closer look at the numbers (ZFW was roughly 43;000 LBS and FOB after the flight was 5;000 LBS) and realized we must have landed overweight. We used normal procedures for calculating landing data without taking into consideration our significantly reduced fuel burn to return to our departure airport. Things are complicated enough with air returns and diversions; but pilots need to be made aware that the fuel burn-off from the flight plan is not valid for calculating landing data in such situations. Perhaps some procedure should be put in place to more accurately compute estimated FOB for landing.