I disagree with your contention that the numbers beyond the actual legal speed for driving are put there as a sales tactic. I think it more stems from human factors in the design of analog needle gauges.
Over a lifetime of experience, people come to expect the zero point of an analog gauge to be at about the eight o’clock position. secondly, you’ll note that on most cars, the speedometer needle is vertical at around 45-65 miles per hour. This makes the speedometer easier to read quickly, allowing the driver to actually keep his eyes on the road while operating the vehicle.
So if the needle is zero at the eight o’clock position, and 60 at the vertical position, in order to make the dial look symmetrical, you have to run the numbers up to 120 or so on the other side.
This convention is not limited to speedometers. Look at any analog needle gauge in your shop, and you’ll see it works the same way: eight o’clock for zero, and more or less vertical for the normal operating zone.
I have a point at odds with that. My speedometer’s vertical point is 80 MPH.
(No, the car won’t go the the 160 MPH max speed on the dial, there is a limiter at 149 MPH.)
Are there really any cars that won’t do 120?
My speedo has the 70 mph mark at the vertical position with a total span of 140 mph. However, like Tardis, the car won’t actually go that fast. It’s top speed is limited to 127 according the websites I’ve seen.
I suspect that the speedos are typically kept symmetrical with the vertical reading simply being 1/2 way through the total range. For most cars that would end up being between perhaps 60 mph and 70 mph. I’ll look tonight at the gages of something capable of higher speeds, like a 370Z, and see where the vertical reading ends up.
There are lots of cars today that can’t do 120. But perhaps not many speedos that don’t go to that high a reading.
I was wrong. Speedos are not always symmetrical.
I checked the Nissan 370Z. Its vertical reading is 110 mph. It’s zero is at about the 6:30 angle and its Top speed about the 4:00 angle. It’s other gages are asymetrical to match.
I remember burying the needle on a hopped-up 70’s duster. I believe that it’s high as a sales tactic, and to keep people from burying the needle so they can’t claim they don’t know how fast they were driving.
Most cars are limited by chip to below their max.
Example: The current Charger is 129, the unlimited police version is 150.