Mercedes-Benz
300 b Convertible D (W 186)

Why it matters

The 300 b Convertible D is the four-door open version of the second series of the post-war Mercedes-Benz 300, the large six-cylinder flagship that later became known as the "Adenauer". It was built for only about sixteen months, from April 1954 to August 1955, and the factory archive counts 136 of them. It has a steel unit body on a cruciform oval-tube frame and seats five or six people.

How rare is it?

Six separate kinds of rarity – never combined into one number.

Each kind of rarity is rated separately and never combined.

Production

Every figure says what it counts.

Independent counts (e.g. engine and colour) are never combined into a calculated figure. Surviving examples are never presented as original production.

History

The 300 range was unveiled at the Frankfurt show in April 1951 and the main series went into production that November. In May 1954 the first revision, internally called 300 b, brought a more powerful 125 hp engine with higher compression, broader brake shoes and a vacuum brake servo. Outwardly it gained swivel windows in the front doors and chrome stoneguards and bumper guards. The archive lists the Convertible D at DM 24,700 in February 1954.

Auction catalogues describe individual cars. RM Sotheby's presents a 1955 car as the factory show car sent to the 1955 Toronto National Motor Show according to its data card, and a right-hand-drive car delivered new to London and first registered there in February 1954, described as allegedly one of seven built that way. The same house gives 146 as the total of 300 b Cabriolet Ds, which differs from the archive figure of 136.

Technical specifications

Engine

Transmission & drivetrain

Chassis, body & dimensions

Performance

“Not yet verified” means no source has been checked for that value yet. Nothing is estimated to fill a gap.

Identification guide

How to tell an original from a replica or later conversion.

The factory type is 186.014 (design name W 186 III). The engine is the M 186 II (186.920), a 2996 cc inline six of 85 x 88 mm rated at 125 hp at 4500 rpm, fed by two Solex 32 PAIAT compound downdraught carburettors with automatic choke. Only a four-speed column-shift manual gearbox is listed for this model. The rear axle is a swing axle with double coil springs and electrically engageable torsion-bar levelling, and the wheels are 5 K x 15 rims with 7.00-15 extra tyres. Visible changes against the first 300 are the swivel windows in the front doors, chrome stoneguards on the rear wings and bumper guards; the later 300 c has a single-joint swing axle, wider rims and larger tyres. An auction car described as a 1955 300 b carries chassis number 186.014.5500359.

Sources

Each number links to the exact facts it supports.

1. 1
   300 b Convertible D / W 186 III, 1954 - 1955 Level A Mercedes-Benz Public Archive (Classic M@RS)

   • Units produced : 136
   • Engine type : M 186 II / 186.920 (Specifications)
   • Number of cylinders / arrangement : 6 / in-line (Specifications)
   • Total displacement : 2996 cc (tax classification: 2975 cc) (Specifications)
   • Bore x Stroke : 85 x 88 mm (Specifications)
   • Compression ratio : 7.4 - 7/5 (Specifications)
   • Valve operation : 1 overhead camshaft (Specifications)
   • Fuel system : 2 Solex 32 PAIAT compound downdraught carburettors with automatic choke (Specifications)
   • Rated output : 125 hp at 4500 rpm (Specifications)
   • Rated torque : 22.5 mkg at 2600 rpm (Specifications)
   • Gearing : 4-speed manual transmission (Specifications)
   • Gear ratios : I. 3.44; II. 2.30; III. 1.53; IV. 1.0; R. 3.08 (Specifications)
   • Final drive ratio : 4.67 (Specifications)
   • Driven wheels : rear wheels (Specifications)
   • Frame design : cruciform oval-tube frame with centre reinforcement / steel unit body (Specifications)
   • Front-wheel suspension : double wishbone (Specifications)
   • Rear-wheel suspension : swing axle (Specifications)
   • Brake system (foot brake) : hydraulic, with vacuum booster, acting on front- and rear wheels; (Specifications)
   • Steering : recirculating-ball steering (Specifications)
   • Wheelbase : 3050 mm (Specifications)
   • Length : 5055 mm (Specifications)
   • Width : 1838 mm (Specifications)
   • Height : 1640 mm (Specifications)
   • Front / rear track : 1480 / 1525 mm (Specifications)
   • Kerb weight : 1820 kg (Specifications)
   • Maximum speed : 160 km/h (Specifications)
   • Acceleration 0-100 km/h : 17 s (Specifications)
   • Fuel tank: position / capacity : in the rear / 72 l (Specifications)
   • Fuel consumption : 12.5 l (Specifications)

   Accessed 2026-11-12
2. 2
   Mercedes-Benz Classic M@RS archive – 300 b Convertible D Level A

   • 136 Convertible D

   Accessed 2026-11-12