For each gear the ratio would be different. Four thousand RPM in first gear might be around 30 MPH, in second gear around 40 MPH, in third gear around 55 MPH, in fourth gear around 65 MPH and in fifth gear around 75 MPH. If you have five manual gears.
You should drive in the highest gear possible at all times. If you can keep the tachometer below 3,000 RPM’s most of the time, you will have your car for a long time.
Otherwise, to avoid speeding tickets, just match the speed of traffic on highways and go slower than necessary on single lane roads.