Quote:
Originally posted by Bob Moseley
..... However, it appears the speedometer was rebuilt utilising the original face but mounted onto a truck or car speedometer head. Whilst this looks good the speedometer head would contain the incorrect gearing.
Thanks for the input but keep commenting.
 Bob
|
Bob, the variations in the "gearing" (eg 3 to 18, 3 to 21 and 5 to 18) are all contained on the rear output of the gearbox, not within the speedo instrument. All Ford speedos, both Waltham and Stewart Warner, have the same internal ratios, as I understand do most US Speedos of the period (incl Jeep, GMC, Dodge, etc Military styles). I think the standard figure is 2000 turns of the cable to an indicated mile. So if the CABLE turns at 2000rpm, the speedo indicates 60mph and the odometer adds 1 mile per min. The different driving and driven gear ratios on the Speedometer output of the gearbox are an attempt to match the rotation of the Differential and wheels (or track!) per mile to the 2000 turns per mile of the speedo.
If this is not clear, I could do some math examples to explain.
Let's assume that our Carrier is trundling along nicely in a straight line at 2000rpm engine speed in 4th gear (1:1 ratio). We will say it has a 5.83 to 1 diff and 35 tooth drive sprocket. Carrier track has a 1.75" pitch. In a 1 min period, the crankshaft will rotate 2000 times, the gearbox output shaft will also rotate 2000 times (1:1 ratio), while the axle shafts will rotate 343 times (5.83:1 diff). If the drive sprocket turns 343 times and moves 35 links per rotation, that is 12005 links per minute at 1.75inches, or about 1750 feet. 1 mile = 5280 ft, so in 1 min our carrier has travelled about 1/3mile, or does a speed of 20mph.
How do we get the speedo to show this? The gearbox output shaft is turning at 2000rpm. With a speedo drive gear of 18 teeth and a driven gear of 3 teeth, the speedo cable will be turning at 666rpm. As the Speedo requires 2000 turns for a mile, 666prm will show 1/3 of a mile a minute, or 20mph! We're on the money, and ready to take on the speed cameras!
