Is there a way to calculate what number of teeth your speedometer gear needs to have ? Case in point. My 56 F100 with T19 ,4 speed has at the time 3.00 gears with P255-70 rear tires. What number of teeth need to be on speedometer gear in trans to make it read accurate. Thanx in advance.
If you know how many teeth your T-19 transmission output shaft has and your speedometer ratio, this may help: http://www.sccoa.com/faq/speedgr.html It might be completely useless, too.
How far off is it now? If you know what the inaccuracy is, you can figure the percentage that it's off. If it reads fast, you have too many teeth on the driven gear. Slow, not enough. If it reads fast by 10%, you need 10% fewer teeth on the driven gear. Keep in mind that older transmissions like yours were designed for a 'range' of tire sizes/rear axle ratios so you may not be able to get a gear with the right number of teeth. Most speedo shops/suppliers can set you up with a 'inline corrector' that goes in the cable that will under/overdrive the cable to make it accurate.
Thanks to both of you. I haven't put it on the road yet. Trying to get close. I have a selection of gears.
Having worked a chassis dyno in several Ford dealers over the years, I got very good at speedo calibrations. The formula burned out gave is good. Keep in mind Ford made speedo driven gears with 15 to 21 teeth. One tooth change works out to about 2 1/2 mph.
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