The formula for determining MPH is:
engine RPM X tire circumerence divided by final drive ratio X gear ratio X 1056
Remember, you were describing how the computer determines what speed is to be displayed on the speedometer.
All the computer needs to know is distance per tire revolution ( for the miles
part), and how long it takes to make one revolution (for the per hour
part). It gets this data from the ABS tone rings.
If what you say is true, the mph would be wrong when coasting downhill with the engine at idle; when the automatic's torque converter is slipping; when either transmission is in a gear other than top gear, (if top gear is what is assumed); etc.
As mentioned in a previous post, people who change gear ratios and don't reprogram, don't see a different error in the speedometer.
A 30" tire travels about 7.85 feet per revolution.
That's about 672.6 revolutions per mile.
If the tire rotates at a rate of 672.6 times per minute, the speedometer will show 60 mph.
I don't see any gear ratio or engine rpm data in that.