Originally Posted by Gkiller

Hello from Texas...new to this forum. I recently purchased a pre-owned 2019 F250 Lariat 6.2L after recently selling my 2013 F250 6.7 Powerstroke. Now on to the issue....when I purchased the truck a few weeks ago, it had just a few miles over 10K on it and looks/runs like new. However, while using my Waze app recently, I noticed that there was a variance in my speedo speed reading and what the Waze speed showed. Generally, the difference ranges from 7-10% , with the higher variance at higher speeds. The truck has stock wheels and tires (verified), so that is not the problem. The other part of this is that the problem is not constant....about 50% of the time, the speed is spot on (in addition to Waze, I also use an app that provides a GPS speed reading that I compare to my speedo speed). After doing a bit of research on the internet, I find nothing other than comments related to the increase of wheel/tire size effecting speed readings and the need for calibration. I then disconnected the battery to see if it could possibly be an ECU issue. After reconnecting, the speedo was right on. However, after parking and then leaving again...the speedo was off again. So the issue is random....sometimes speed is on, sometimes off.
I've called and spoken to 2-3 service advisors at local Ford dealerships, and none say they have heard of this issue. One stated that it is possible the prior owner may have installed larger tires, had the truck recalibrated for the larger tires, and then before trading it in put the original tires back on but did not have the truck calibrated back to the original tire size. That might make sense....if the speedo was consistently off at all times. But again...the problem is intermittent.
Any ideas or recommendations? I really do not want to take my truck to the dealership and be without it for the "several days" that each advisor told me it would take for them to diagnose and test to try to determine the problem. My experience has been that "several days" usually means a week or two based on my prior dealings with the SD's.

Just looking for input on potential causes, as I have found that the feedback from forum members is typically much more helpful than that of those at dealerships. It's not that big of an issue, as I just turn the GPS speed app on and watch my speed using it when driving, but I know there is going to come a time when I don't do that and get nabbed and a ticket for speeding.

Any constructive input or advice would be appreciated!