Thread: Tire Size
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Old Fri, June 18th, 2010, 08:53 PM
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Originally Posted by BigSur View Post
I was looking at the truck speed gauge not Gryphon, so maybe that's right. Why is there a discrepancy b/w the Gryphon and Truck speedometers? I thought the Gryphon was supposed to correct the truck gauge when TS is accounted for. But yes, I'm definitely off .1 mile w/in a very short distance, so I think I need to crank it up a bit. Thanks guys!!!!
Your truck speedometer is an analog device. It receives digital data from the PCM, the same that is fed to the Gryphon and the digital odometer. In the conversion to analog, mechanical gear/spring errors cause the difference. I suspect that Ford insured the speedometer would read slightly fast so they could never be sued for someone getting a speeding ticket due to a speedometer reading slow. That's a guess, of course, but if there has to be an error, I'd expect Ford to "bias" it in that direction. Because mechanical devices can't really be dead accurate, speedometers in different vehicles are going to have slightly different errors.

But, there's no "conversion" for the odometer and the Gryphon speedometer. You should be able to get them to tell you something that is very close to the truth.

However, all this is a bit like counting angels on the head of a pin. Variations in tire construction, tire height/width profiles, tire pressure, vehicle loading, tire "slippage", and vehicle speed will ALL effect the accuracy of both the digital and analog reading. The PCM essentially counts axle revolutions, through a sensor. It then "calculates" speed and distance based on the number of revolutions it expects the axle to make in a mile (which the Gryphon inputs after calculating it from your tire circumference (or tire diameter if you have a CS/CTS). You should be able to get close, probably within 0.1%, but much more than that is just wishful thinking. (0.1% is 1 mile in 1000 miles).

As shotgun said: "Welcome to the unofficial (but highly popular) TS Calculation Club." I'll add, "No matter what you enter, you'll be wrong". But, it sure is fun!

- Jack