Section 171.12 of the Minnesota Statutes deals directly and specifically with the MN driving records.

Subsection 2 states that the department shall file all accident reports and abstracts of court records of convictions and violations received by it under the laws of this state and its political subdivisions, and in connection therewith maintain
convenient records or make suitable notations in order that an individual record of each licensee showing the convictions of such licensee and the traffic accidents in which the licensee has been involved shall be readily ascertainable and available for the consideration of the department upon any application for renewal of license and the revocation, suspension, or limitation of licenses.

Subsection 3 notes that the driver's record pertaining to revocations, suspensions, cancellations, disqualifications, convictions, and accidents is cumulative and must be kept for a period of at least five years. Furthermore the driver's record pertaining to violations of a driver or vehicle out-of-service order must be kept for a period of at least ten years.

The driver's record pertaining to felony convictions in the commission of which a motor vehicle was used and certain alcohol-related offenses and licensing actions listed are permanently retained on the record.

So it would appear that a speeding ticket would normally stay on your Minnesota driving record for ten (10) years. There are exceptions though.

Subsection 6 of Section 171.12 section may relate to your speeding ticket. This subsection is titled Certain Convictions Not Recorded and here it states:

(a) Except as provided in paragraph (b), the department shall not keep on the record of a driver any conviction for a violation of a speed limit of 55 or 60 miles per hour unless the violation consisted of a speed greater than ten miles per hour in excess of a 55 miles per hour speed limit, or more than five miles per hour in excess of a 60 miles per hour speed limit.
(b) This subdivision does not apply to (1) a violation that occurs in a commercial motor vehicle, or (2) a violation committed by a holder of a class A, B, or C commercial driver's license, without regard to whether the violation was committed in a commercial motor vehicle or another vehicle.

For more information on your driving record or on MN driving record laws contact the MN DPS.