"Mr. Ratburn's Convertible" is not an official name.
Mr. Ratburn's Convertible is a red 2-door convertible that is driven by Arthur's third grade teacher, Mr. Ratburn.
Mr. Ratburn's car is red on the exterior and has round headlamps and red oval taillights. The roof is tan and is usually open, the interior is tan with leather bucket seats and a center console mounted automatic transmission gear shift, and the dashboard is the red like the exterior.
In a thinking sequence in the episode Prunella's Special Edition, Mr. Ratburn's car gets picked up by the Mangling Maple (a parody of the Whomping Willow from the Harry Potter series)
In Best of the Nest, Mr. Ratburn also has a van. However, this van only appears once in the series.