You care more about the gear in the transmission side. The speedometer is fix at 1000 revolutions per mile.
So you adjust the gear in the sender to match the ratios that take into account the tire sizes, rear end gearing and revolutions of the shaft to that number.
There are some apps that allow you to input the data.
Or you can post what transmission your have. What is your tire size and what is your rear end gear ratio.
The bouncing would have definitely been a busted cable or binding which you fixed with replacement.
