Engine speed=(MPH)(Ratio)(336)÷(Diameter)

or

MPH=(Engine speed)(Diameter)÷(Ratio)÷(336)


Please be aware that the formula works very accurately for a manual transmission. The automatic has slip so you must add a couple of hundred RPM to the result you get for engine speed in the first formula and you must subtract a couple of hundred RPM from the tach reading when you use the second formula.

For the 123 with a 25.6 tire diameter and a 3.46 ratio, the engine speed at 70 mph with the automatic should be about 3375 rpm. So, it appears that the tach correlates well with the speedometer.