On my 01 nissan altima, the gauge read 20mph over the real speed, I replace the speed sensor and the speed still the same ? what Im missing or what do i have to check

If the sensor and wires check good the gauge is bad.

This link may be helpful.
Bob

https://www.google.com/url?sa=t&source=web&rct=j&ei=jVO4VK_hEMvbsAT83YDgCA&url=http://www.autopartswarehouse.com/shop_years/nissan-sentra-speed-sensor-2001.html&ved=0CDcQFjAA&usg=AFQjCNHmI3zYnmpPLAMrWGUYw4Hq3dw2tQ&sig2=NZHLkkBawd1OUJEZgAwLQQ

The speed sensor is probably disconnected or the wiring was damaged. The CPU needs this input to demand shifts.

I’m happy to assist further over the phone at https://www.6ya.com/expert/samuel_dda9c48bb7ffbfd3

Mass air flow not MAP ?
Nissan Infiniti Mass Air Flow Sensor Testing Replacement P0100

Check yhour thermostat,might not be opening all the way and making your Nissan a hot tamalie.

you need acrankshaft position sensor.while your at it replace cam sensor.same exact sensor same price and same high rate of failure.for another 60 bucks you'll save yourself alot of grief.

Remove wire at fueltank end, if your gauge drops back to the bottom you have a faulty sender unit in your fuel tank, it is removed from where you just removed wire from, ensure your fuel is low before opening seal or you will waste fuel when you open it.
If your gauge still reads high after removing wire from fuel tank sensor, the fault lies with the wire itself earthing out somewhere along its length.
Have fun.

What is your temperature gauge reading when this is happening with your car if you have a gauge see what it says.

Take it back to where you had it serviced. They probably simply missed connecting the fuel level sensor. The engine light is probably from a missed emissions hose that goes to the gas tank.

the problem was not before but is after engine work
suggests harness connection , pinched wires to the inlet manifold
problem throttle control sensor or possible an ECM corruption
try using a scan tool to reset the ECm