Yes, you are right, and I was right. We're both saying the same thing, from opposite directions.

What I meant, is that as you decrease wheel diameter (i.e. tire wear), your "actual" speed would decrease congruent to the wear when compared to a fixed or "target" displayed speed, assuming the speedometer is working on a static algorithm or mechanical/electronic trigger that is directly calibrated to gearing, as most speedometers are.

Now if the speedometers were using GPS or radar technology for determining speed, gearing and tire wear would be irrelevant.

If you buy the car new, with new tires, and when your speedometer reads 80mph, you are actually travelling 80mph, then I would ball-park estimate that once you've worn the tires to almost-bald, you're actually only travelling 78 mph with the needle on 80 on the dash.

BTW, if you want to hear something funny, the speedometer on our 1911 White Touring works like this..

A cable, with a gear on the end of it, engages a sprocket on the LH front wheel. At the other end of the cable, at the dash, the cable spins a little FAN that blows air into a tiny metal horn, that is attached to the needle's axle, which is spring-loaded. The harder the fan blows air into the horn, the further the needle is pushed around the gauge. The calibration is all in the pitch of the fan, and the shape of the horn!

Amazing how things that seem so "standard" to us today, were being done in any number of crazy ways back when everything was a new invention, somebody's unique "idea" of how to accomplish something.