Here are links to good speedometer calibration methods:
https://www.ford-trucks.com/forums/s...hreadid=179955
https://www.ford-trucks.com/calculat...trument_error/

And a speedometer calibration re tire size re... calculator:
https://www.ford-trucks.com/calculat...php/p-mph_rpm/

More calculators:
https://www.ford-trucks.com/calculators/index.php


FTE also has CLASSIFIED ADS for buying and selling vehicles and parts.


Formula for instrument error:
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Calculator here:
https://www.ford-trucks.com/calculat...trument_error/

Manual methods for old folks:
actual mph = 3600 / seconds per mile

speedo error percent =
difference between actual and indicated speed / actual speed x 100

indicated distance =
odometer reading at finish - odometer reading at start

odo error percent =
difference between actual and indicated distances / actual distance x 100


Seconds to travel ONE measured mile:
==================================
Time= Actual road speed
50=72
51=71
52=69
53=68
54=67
55=65
56=64
57=63
58=62
59=61
60=60
61=59
62=58
63=57
64=56
65=55.4
66=54.5
67=54
68=53
69=52
NOTE- Time your vehicle over a 10 mile distance for ease and accuracy.


Speedometer gear replacement:
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REMOVE the original driven gear.

Ts = Number of teeth on service driven gear.
To = Number of teeth on original driven gear.
Va = Actual vehicle speed.
Vi = Indicated vehicle speed.
VSS = Vehicle Speed Sensor.

NOTE: Design accuracy is: -2/+5 mph at 30 and 60 mph
(-3/+8 km/h at 50 and 100 km/h).

NOTE: Use one of the following formulas:
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With Va, Vi, and To known, calculate new number of driven rear teeth.
Ts = (Vi x To)/Va
Ts = (To x new tire revolutions per mile)/(old tire revolutions per mile)
Round to whole number, dropping fractions less than .6, and rounding up if
fraction is .6 or higher.