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 The 'AB' and 'ABF' prefixes were issued by Ford in very early 1932, and were quickly superseded by the typical 'B' and 'BF' prefixes.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, small displacement 14.9 HP (R.A.C.) engines produced by Ford Motor Company, Ltd. in England.

Ford
Part Number(s)
Pinion
Teeth
Ring
Teeth
Gear
Ratio
Notes
Aftermarket 11 36 3.27 : 1 Produced by Steve Becker @ Bert's.
This is the highest speed gear ratio available.
Best suited for light cars, and/or flat terrain, and/or higher horsepower engines.
Aftermarket 11 39 3.54 : 1 This gear ratio was used in post-1934 Ford production in a V8 straddle-mounted pinion format, but is not found in Ford Parts Lists in a Model A or B format.
This ratio has been available aftermarket for many years and is considered a "high speed" gear set.
Best used with light cars, and/or flat terrain, and/or higher horsepower engines.
A-4209 10 37 3.70 : 1 Early 1928 production non-threaded pinion and ring set.
  • A-4209 Production set was 3.70:1 ratio.
  • A-4209-AR Service Replacement set was 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.
A-4209-B
AB-4209-A
B-4209-A
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.
This gear ratio is often used with the 26% Mitchell Overdrive unit, or the 27.6% OD 1984-87 F-150 Tremec RTS 4-speed transmission conversion.
A-4209-D
(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 ?)
A-4209-C
AB-4209-B
B-4209-B
9 37 4.11 : 1 Typical Model A and B threaded pinion and ring set.
  • Optional Model A ratio, typically found on heavy cars, such as Pickups, Fordor Sedans, and Station Wagons, or in mountainous terrain.
  • Standard Model B ratio.
For best overall drivability, this gear ratio is recommended for use with: the 26% Mitchell Overdrive unit, or the 27.6% OD 1984-87 F-150 Tremec RTS 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 (R.A.C.) British Model AF Fords.
AF-4209-B
ABF-4209
BF-4209
9 41 4.56 : 1 Typical Model A and B threaded pinion and ring set.
  • Standard ratio used with 14.9 HP (R.A.C.) British Model AF and BF Fords.
AF-4209-D
(duplicate of
AF-4209-B ?)
9 41 4.56 : 1 Appears in some foreign Model A Parts Lists for use with 14.9 HP (R.A.C.) 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 information on Ford Garage:

  1. For more Model A & B related information, use the Site Search box at the top or bottom of this page.
  2. Model A & B Original 5-Z-1833 and 5-Z-2065 Pinion Nut Wrenches
  3. Model A Mitchell Synchronized Transmission
  4. Model A Ruckstell Dual High Transmission

April 2009