Digital speedometer question

I've been looking at those obdii digital speedometers on Amazon and was wondering if anyone has experience with those and if it will actually be accurate. I run standard size tires.

 

Factory tires, sure. Taller tires, no.

My iPhone speedometer is pretty accurate

 

If you want that do it right and get a scangauge or an ultra gauge. I’ve got an ultra gauge mounted to the drivers handle, it can be set to show whatever live engine data id like. Mine usually shows coolant temp, mph, and mpg and a couple other things I’m forgetting. Having the ability to read any engine data live is priceless when troubleshooting issues. And immediately seeing what code is on if the cel turns on.
It will be accurate to factory tire size. Won’t help with speed correction from oversize tires though.

 

Thank you for your suggestions, I will look into that, I was skeptical of the OBD2 port not updating fast enough to display speed

 

It updates somewhat fast, at least on the UG its probably refreshing every 1/2 second or so. Slow enough to notice, but fast enough that it doesn't actually matter. Not like the truck can accelerate fast enough for that to be a problem. No clue if scangauges are the same or better or worse.

Usually notice the refresh rate delay the most when hitting the gas going up a hill, and having to wait half a second for the live MPG to drop and reflect the change in engine load