Your speedo drive gear cannot have a fractional number of teeth. Its either a 6 or a 7, there isn't any other option. Look at one end of the gear: how many teeth are there showing? For your 3.00 rear end I am pretty sure it is a 7 tooth.
I am not sure exactly what you have been reading but this problem has several aspects. What was the calibration when it left the factory and what is the calibration today? They aren't necessarily the same unless you have paid someone a few $$ to calibrate your speedo lately. Most of us don't do that but instead buy the $5 speedo gear that gives us the nearest-to-correct readings. There is also the problem of what size tire are you using.
The goal was to have 1000 revolutions of the speedo cable equal one mile. That was to make the odometer correct as well as the speedo. To make that happen Ford offered speedo gears with 15-24 teeth. Today NPD only sells 15-21 so you are somewhat limited. Looking in the Ford parts catalog for 1966 it says that a Mustang sized tire (actually only 25.5" diameter) with a 3.00 rear end needed a 17 tooth gear for either a C4 or a 4-speed. (For a 3-speed manual they said it was a 19 but that makes no sense at all. All Mustang transmissions of the day were 1:1 in high gear.)
Calculating factory tire sizes, etc., with a 7 tooth drive gear (the standard one) and a 17 tooth driven gear gives a speedo RPM of 976 at 60 MPH. That is not exactly the correct 1,000 but its only off by 2.4%. Changing from a 17 to either a 16 or 18 tooth gear changes the result by about +-6% so you couldn't get any closer than using a 17.
Does that mean your car should be using a 17? Not necessarily but probably. I have a '66 GT with 3.00 rear end and to make my speedo read correctly I would need a 17.4 tooth gear using 195/75-14 tires (as close to original diameter as possible). I use a 17 and get a reading of 61 when I am doing an actual 60. By the Ford numbers I should need a 16.6 tooth gear to be accurate (pretty close to your 16.8). 16.6 and 17.4 both round to 17 but my speedo calibration is not quite 1,000 RPM. Do I care? Not really.
You need to measure how far off your speedo is now with whatever gear you are using then calculate how many teeth you need to make it accurate. You can use mileposts on the freeway with a stopwatch or sometimes you can find a radar speed sign that will give you a speed reading directly (not always very accurate though). GPS can also give you an actual speed reading. Compare those to what your speedo is telling you. None of the calculators may be accurate for your car unless its speedo has been officially calibrated to 1000 RPM. If you are using a 17 tooth gear and are within 2-3% then that is as good as its going to ever be.
I am not sure exactly what you have been reading but this problem has several aspects. What was the calibration when it left the factory and what is the calibration today? They aren't necessarily the same unless you have paid someone a few $$ to calibrate your speedo lately. Most of us don't do that but instead buy the $5 speedo gear that gives us the nearest-to-correct readings. There is also the problem of what size tire are you using.
The goal was to have 1000 revolutions of the speedo cable equal one mile. That was to make the odometer correct as well as the speedo. To make that happen Ford offered speedo gears with 15-24 teeth. Today NPD only sells 15-21 so you are somewhat limited. Looking in the Ford parts catalog for 1966 it says that a Mustang sized tire (actually only 25.5" diameter) with a 3.00 rear end needed a 17 tooth gear for either a C4 or a 4-speed. (For a 3-speed manual they said it was a 19 but that makes no sense at all. All Mustang transmissions of the day were 1:1 in high gear.)
Calculating factory tire sizes, etc., with a 7 tooth drive gear (the standard one) and a 17 tooth driven gear gives a speedo RPM of 976 at 60 MPH. That is not exactly the correct 1,000 but its only off by 2.4%. Changing from a 17 to either a 16 or 18 tooth gear changes the result by about +-6% so you couldn't get any closer than using a 17.
Does that mean your car should be using a 17? Not necessarily but probably. I have a '66 GT with 3.00 rear end and to make my speedo read correctly I would need a 17.4 tooth gear using 195/75-14 tires (as close to original diameter as possible). I use a 17 and get a reading of 61 when I am doing an actual 60. By the Ford numbers I should need a 16.6 tooth gear to be accurate (pretty close to your 16.8). 16.6 and 17.4 both round to 17 but my speedo calibration is not quite 1,000 RPM. Do I care? Not really.
You need to measure how far off your speedo is now with whatever gear you are using then calculate how many teeth you need to make it accurate. You can use mileposts on the freeway with a stopwatch or sometimes you can find a radar speed sign that will give you a speed reading directly (not always very accurate though). GPS can also give you an actual speed reading. Compare those to what your speedo is telling you. None of the calculators may be accurate for your car unless its speedo has been officially calibrated to 1000 RPM. If you are using a 17 tooth gear and are within 2-3% then that is as good as its going to ever be.
