Speedometer lying

Man said that this issue somehow affects engine performance,so he can't just ignore it and look at gps speed.

 

How did you fix it? I am getting the same problem. My speedometer is fluctuating and it is effecting my cruise control and engine brake and engine performance.

 

Have you changed drive tires?

And which transmission?

Could be that you have a good speed sensor but... slop in output of trans causing erratic hall effect reading once warmed up enough. Tolerances can do weird weird things to sensors accuracy...

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I drive a mechanical and switched from 24.5's to 22.5's My speedometer is off by 5 mph, meaning when it says I am going 65, I am really going 60. I have verified by using the markers on the side of the road but found out about it when I passed multiple speed signs in construction zones. I assume it is because of the tire size change. Wondering if there is a speedo gear in the trans somewhere, like on the old cars.

 

Can of worms here. If your throttle is fly by wire, ie digital binary then your speedo is probably a digital.

 

what year is the truck? mechanical engine does not mean mechanical speedo. cable speedo's have been gone a good while. there are micro switches on the back of the speedo to calibrate,and the charts are available online. simple fix once you have the switch combo.