Old thread, but still valid.
My 2 cents...indicated speed vs actual speed...a discussion amongst motorcyclists and racers for decades.
As an avid motorcyclist, (and car owner) I know that wheel
diameter will affect indicated speed, as all vehicles speedos are calibrated with the tires they came with new. All the manuals state that changing tire size will affect...albiet slightly in most cases, the indicated speed.
I have yet to find a bicycle speedometer that reads exactly the same as a GPS speedo...Strava/Cyclemeter/Giant app.
I also have one of those flashing speed reminders on the last leg of my rides, and It's my bit of fun trying to get over 45kph...my legs always tell me what my odds are ;-) ...but yes, even that reads lower than the speed on my display, and different yet than my Cyclemeter (my go to app) but I don't sweat it.
It matters little to me which one is correct, I doubt i'll be attracting "officer Friendly" anytime soon lol.
Tire size...who cares? you have an E-bike!...I run those 50's like a boss and get loads of kms per charge on Eco+...and...on the flats oe slightly downhill, I can easily maintain over 32kph...when the rise begins, I technically have 44 options to chose from, so I don't fret