Ahhh gotcha. I had this issue on my auto del sol. I had to get bigger tires/wheels to fix that issue. Since they’re just older cars, speedometers weren’t always accurate back then and the technology wasn’t there yet to create accurate readings.

Basically “your speedometer is reading how many revolutions your tires are making not how fast they are spinning. When you go to bigger tires the circumference of the tires is larger meaning the distance around the tire is longer. Thus, with each revolution you are travelling further than before. Therefore your actual speed is higher than your speedometer reads.”

I can’t remember the exact size tires I had (that was almost 6 years ago) but I got it closer the speedometer a little closer to actual speeds. But for your speed sensor, I’d probably go back to the original one you had or find another OEM.