Stewart # 26 Speedometer Calculations

As I mentioned in an earlier post, I acquired a period correct speedometer off of eBay. It is a Stewart & Clark #26 Speedometer.

The other part is a Stewart & Clark Swivel Drive that once mounted correctly will allow the front wheels to turn on its axis and at the same time allow the cable drive to the speedometer to be mostly stationary. To make all this work, I will have to modify the right front brake disc by machining spur gear teeth on its perimeter. The following are my calculations for achieving this added accessory.

Knowns:
Stewart #26 Speedometer requires:     1009 revolutions/mile
Front Disc Brake Outside Diameter:     8.66 inches
Gear Specs:                                        Diametral Pitch (DP) of Gears: 8
                                                   Pressure Angle: 20° (Specs from Ford Model T)

Circumference of Tire:                                  

Outside Diameter (inches) x p        27.2 (in.) x 3.14159/12 (in./ft.) = 7.1209 ft.
               12 (inches/ft)

Tire Revolutions per Mile:             5280 ft./7.1209 ft. = 741.479 rev./mile

Disc Brake Gear Spec (DBG)

# Teeth = (DP x Outside Diameter)-2   (8 x 8.66)-2 = 67.28 teeth
Therefore closest gear spec would be:  67 teeth
                                                               Diametral Pitch = 8
                                                               Pitch Diameter = 8.375 inch
                                                               Outside Diameter = 8.625 in. 
                                                               (only 0.0175 inch off the radius)
Speedometer Drive Gear Spec (SDG)
Stewart #26 requires 1009 rev./mile
The speedometer cable swivel reduces the rpm by a 2.5 to 1 ratio.
Therefore the Speedometer Drive Gear (SDG) must rotate 2.5 x 1009 rev./mile = 2522.5 rev./mile

SDG # Teeth = (DBG (rev./mile)) x( DBG (# Teeth))   =

                                       (SDG (rev./mile))                                          

                  (741.479 (rev./mile)) x (67 (# Teeth)) = 19.69 teeth
                                  2522.5 (rev./mile)

Therefore closest gear spec would be: 20 teeth

                                                             Diametral Pitch = 8
                                                             Pressure Angle = 20°
                                                             Pitch Diameter = 2.500 inch
                                                             Outside Diameter = 2.750 inch

I believe that the Stewart Speedometer will be within 1.5% accuracy with the above. It will be fun to see if my calculations and assumptions work out. Regardless I believe it will be close enough.

I have attached drawings/sketches of the gears through these links:

Speedometer Drive Gear Sketch

Disc Brake Gear Modification

Follow this link for the: Stewart #26 Speedometer Swivel Drive Mount