I think my speedometer is off by 7mph. I used multiple Heads Up Displays (HUD) on my phone to check the speed. Each HUD app showed 7mph faster than the dash speedometer. I have the stock size tires on it. Also checked by mile markers, seemed a little off.

My speedometer has the mechanical odometer, and was wondering how to adjust it. Seen online how to adjust speedometers with digital odometers, but not mechanical odometers.

87 F150, Automatic Trans, Custom Cab, 5.0L/302 EFI.

 

You have to change the speedometer drive gear.

 

I have not, I did put new tires (vehicle spec that was on the door info). It did have larger tires by previous owners (all used, different type and dry rotted), but did not check with those tires. Not sure if they changed the gears or not. I bought it, and I had to work on it before I was able to drive. Once I got it running, I put new tires on.

 

Not talking about the differential gears, you need to change the speedometer driven gear to one with a different number of teeth. Call these people:

https://transmissioncenter.net/shop/...r-drive-gears/

 

Yes, That is what I though you meant. I'm not sure if one of the previous owners before me added the larger tires and change the speedometer driven gears.

 

Well, you need to pull the gear and count the teeth and go from there.