Speedometer vs actual change after 40 mph

Have noticed with my 2007 that in the upper speed ranges (above 40) there is a difference of up to 3-4 mph.
I had known there was some kind of difference when I was on a road trip with two vehicles and when both were set at the same speed, but the other vehicle always inched ahead.
I started getting a bit more curious after I did a brake/rotor/bearing replacement. Wanted to make sure the speed sensors were working right. The back circular shield was spot on/dead center for the speed sensor opening, but I would also think if there was an issue there it would happen for all speeds, not just above 40.

But I was seeing the same symptoms. I have a GPS app on my phone that gives a really accurate reading, and it's been verified/matched with other vehicles at corresponding speeds.
So for mine, If I'm going along at 40-45 or so, I can't see too much of a difference between the speedometer an the app. I can't quite remember the ratio of difference after 40, but my limit is about 75 for highway around my area. At 75 the app reading says 72 or 71.
In the past when I first asked about a speed difference, I thought people had mentioned that a 1-2 mph difference cane be expected. But this is getting into the 3-4 mph difference.
I'm the primary driver of the vehicle so for me to mentally adjust speed isn't that big of an issue, but I'm borderline curious/concerned what would cause 3-4 mph in speed difference.

Any thoughts?

 

I guess I'm not trying to say my speed should be perfect in an absolute measuring to whatever baseline, but just the fact that it's off this much makes we wonder. Never had a vehicle with this much discrepancy so it's just bugging me trying to understand why it's doing this.

 

err, I just had new tires installed and I believe the default 185 size was installed.
I do remember someone else mentioning the 195 as an option, but I think last time I asked about that the shop folks wouldn't entertain the idea since that wasn't the specified size for the car.