Why does my nissan sentra have truoblepicking up speed over 40 mph

my car runs but has trouble picking up speed

The purpose of a transmission is to give you more power when you accelerate from a full stop and save fuel when you get up to speed. The lower gears will always increase speed with more zip than the higher ones. The question here is whether what it's doing is abnormal.

If it has good power from zero to 40 that says the motor is working properly and, if there is a problem, it's likely with the transmission.

If you feel anything at the lower gears that indicate it's not performing there either, the first thing to look at is spark plugs and timing AKA a "tuneup." There are other, more serious possibilities but always check the simple stuff first.

A car with tune up problems will run rough, use more gas than it should and have less power.