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This is a custom made 40ft 5th wheel trailer. Such trailers are often used by Hotshot type trucks. Pulled by lite duty trucks like the Freightliner FL-50, FL-60, FL-70, or one of the mid duty GMC Kodiac trucks, or something similur from Boley/ Walthers.
The wheels are 5.20mm or 18.5 inch HO scale diameter, and the tires are 8.44mm or 30 inches HO scale diameter. Deck is 139.4mm or 40 feet HO scale long, and 29.45mm or 102 inches HO scaling wide.