Speedometer correction

There have been many threads at Ford-Diesel.com and Ford-Trucks.com concerning speedometer correction. Since the Superdutys use a tone ring to calculate speedometer information this has led to some confusion when tire sizes or ring & pinion gears are changed. I've made a couple pictures and will try to explain the differences between older trucks and the Superduty as far as speedometer readings.

Figure 1 is the setup most people are familiar with. All my previous vehicles have been this way. The transmission or transfer case tailhousing contains a gear which mechanically or electrically drives the speedometer. The speedometer gear is setup at the factory to correspond with the ring & pinion ratio and tire size installed at the factory. If you change the ring & pinion ratio and/or the tire size the speedometer will not be correct. The 'miles per hour' formula shows how the ring & pinion ratio and tire diameter are both used in determining vehicle speed.

Figure 2 is the setup for the Superdutys. The computer reads rpm information from the tone ring which is attached to the ring gear (and therefore turns at the same rpm of the axle). As an example...if the computer reads 1000 pulses (teeth) per second, and the tone ring has 100 teeth, the computer knows the axle is turning at 1000/100 = 10 revolutions per second. The Superlift Truspeed corrects the speedometer by adjusting the pulses being reported to the computer.

Another way to correct a speedometer is to have the dealer reprogram the computer for a different value for 'tire revolutions per mile'. A limitation of this is that the factory computer is limited to a value of 601, or approximately a 35" tire.

Many people have a hard time understanding why changing the ring & pinion ratio doesn't affect the speedometer. From the above picture it can be seen that the speedometer information is taken after the ring & pinion, so the ring & pinion ratio doesn't affect the speedometer reading.

Disclaimer on Figure 2: I don't know if the computer reads the values every second. This is just an example, but the principle is the same.

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