Speedo gears are the easy part and part which have to be matched to the basic tire size/rear axle ratio information. THAT gets the odometer to read correctly ONLY. For example, the odometer on my '67 Newport is dead accurate according to the Interstate Mile Markers on a 10 mile run at 60mph (60 secs/mile = 60mph). BUT on that car, the speed readings are 10% higher than they should be. How might that happen? The magnet in the speed cup on the speedometer head needs adjustment.
The speedometer cable turns a bar magnet in a "speed cup" near where the speedometer cable attaches. That magnetism turns a "mating" soeed cup which is attached to the speedometer needle shaft. As one spins, it takes the other one with it. With the second speed cup turning against a clockspring attached to the speedometer needle shaft (which returns the needle toward "0" as speed decreases). The speed reading is a function of the magnet's strength or lack of. Too much and the speed is high. Too little and the speed is lower. ALL for a given speedometer cable turning speed.
At speedometer shops, they had either a bench-located calibration mechanism. Place the speedometer head on it, engage the drive mechanism, turn it on and increase the speed as you watch how the calibration mechanism and the vehicle's speedometer head's needles increase or decrease together. THEN, to get the speed calibration more correct, a "zapper" tool was used to increase or decrease the bar magnet's strength to get a more correct speed reading. Usually, the spec range of speeds would err on the slower side of things. I have seen this done, in the past, and it is a purely "cut and try" method! One short zap and speed decreases. Another short zap and the speed increases. Hopefully a third short zap gets it within a mph or so, plus or minus.
Knowing these things, it is a mystery to me how my '67 Newport has an accurate odometer and an incorrect speed reading . . . considering how difficult it is to remove the speedometer on those cars. So, I'm more concerned about an accurate odometer to check mpg and can compensate for a very optimistic-reading speed reading by mentally compensating for this.
As for conversion of the '67 or so tire sizes to more modern metric sizes, take the old "inches" section width number on the tires listed in the FSM and multiply them by 25.4 to get the approximate metric tire size in "75 section width". As for the alpha-numeric sizes on C-bodies . . . G78 would be P215/75, H78 would be P225/75 (which is actually a bit closer to the old J78/78, but works well), and L78 being P235/75 (which can also work for the short-lived L84). The P235/75-15 is also available in "XL" extra load (6-ply rating) variations, if needed.
Now, back when the cars were newer, many complaints of "inaccurate speedometer" (due to traffic tickets or such) were usually addressed with speedometer driven gear (where the cable attaches to the transmission tailshaft housing), going up or down one tooth, depending upon how far off the speed reading allegedly was. But it seemed that most of these things were more related to GM products than Chrysler products, back then. Nobody came back with "My gas mileage dropped after y'all did my speedometer work" complaints, though, not sure how the speedometer work affected how well their engine ran. LOL.
Sorry for the length. Hopes this might explain how that system works, plus my own experiences.