Model A & B

Ford Garage

Rear Axle Gear Ratios

The following table shows the various Ford part numbers and gear ratios for the matched ring and pinion gear sets used on 1928 through 1933 Model A (103-1/2" WB) and Model B (106" WB) Ford car and commercial chassis, not trucks.

In some cases both 'A' and a 'B' prefix part numbers are shown. This is because the original Model A part was physically carried over into 1932 and later production usage, but with a new 'B' part number prefix. The actual physical parts are identical.

As a matter of Ford process, all Model A parts which were also used on the Model B were renumbered by Ford in 1932 with new prefixes, and starting with no suffix. The 'AB' and 'ABF' prefixes were issued by Ford in very early 1932, and were quickly superseded by the typical 'B' and 'BF' prefixes.

The 'AF', 'ABF', and 'BF' prefix gear set ratios were used with the small bore, low displacement engines produced by Ford Motor Company, Ltd. in England.

Part Number(s)
Aftermarket 11 36 3.27 : 1 Produced by Steve Becker @ Bert's
Best suited for light cars, flat terrain, and/or higher horsepower engines.
This is the highest ratio available.
Aftermarket 11 39 3.54 : 1 This gear ratio was used in post 1934 Ford production in a V8 straddle-mounted pinion format, but was never released by Ford in Model A or B format.
This ratio has been available aftermarket for many years and is considered a "high speed" set.
A-4209 10 37 3.70 : 1 Very early 1928 production non-threaded pinion and ring set.
A-4209-AR Service replacement set was changed to 3.78:1 ratio.
A-4209-AR 9 34 3.78 : 1 Early 1928 production non-threaded pinion and ring set for Service replacement use.
9 34 3.78 : 1 Typical production Model A and B threaded pinion and ring set.
  • Standard Model A ratio.
  • Optional Model B ratio, typically found on light cars.
(duplicate of
A-4209-B ?)
9 34 3.78 : 1 Appears only in the October 1929 US Parts List.
Likely a typo. (duplicate of A-4209-B ?)
9 37 4.11 : 1 Typical Model A and B threaded pinion and ring set.
  • Optional Model A ratio, typically found on Pickups, Sedans, and Station Wagons.
  • Standard Model B ratio.
For best overall drivability, this ratio is recommended for use with the 26% Mitchell Overdrive, or the 1984-87 F-150 Tremec RTS 27.6% OD 4-speed transmission conversion.
Aftermarket (?) 9 39 4.33 : 1 This gear ratio reportedly exists in early 1932 production.
This ratio was used in later (post-1934 Ford V8 production) in a straddle-mounted pinion format, but is not found in Ford Parts Lists in a Model A or B format.
May have been produced aftermarket for Model A/B at some point in history.
AF-4209-AR 9 41 4.56 : 1 Early 1928 non-threaded pinion and ring set.
  • Standard ratio used with 14.9 hp British Model AF Fords.
9 41 4.56 : 1 Typical Model A and B threaded pinion and ring set.
  • Standard ratio used with 14.9 hp British Model AF and BF Fords.
(duplicate of
AF-4209-B ?)
9 41 4.56 : 1 Appears in some foreign Model A Parts Lists for use with 14.9 hp British Model AF Fords.
Likely a typo. (duplicate of AF-4209-B ?)

Some ratios listed above have the same pinion or ring gear tooth count as other listed ratios, however the individual gears themselves are neither the same nor interchangeable!

Ring and pinion gears were individually matched and lapped to each other for quiet operation. NOS matched gear set pairs usually had a matching three digit serial number electrically or mechanically engraved freehand somewhere on each gear.

Additionally, NOS gears usually have traces of orange chalk on the teeth. The chalk was used during the matching/lapping process of the gear tooth contact. The chalk should not be mistaken for surface rust.

In some instances the ring and pinion tooth count can also be found stamped on the banjo housing web or flange, as show below.

More related web pages on Ford Garage:

  1. Model A & B web pages, use the Site Search box at the top or bottom of this web page.

April 2009