my 2000 lincoln ls cruising at 40mph my rpms or 2300 what could be the problem

does the clutch slip when you take off?
Maybe your clutch is fried!

l had one years ago and what a mess. l finally realized it was just a Fix Or Repair Daily.

if an auto trans , it may be slipping and in need of repair

If it is an automatic transmission perhaps it is not going into overdrive. Count the number of times it shifts, it should shift 3 times after you start moving.

l guess that it has not switched on the 3th gear. Tell us how many times it changes gears till you're at 40mph.
Maybe you should change oil and filter. Or see the electronic (overdrive disabled ?)

the transmission did not shift into high.. check the fluic level.

if its auto, at 40 mph 4th gear likely has not kicked in nor has the torque converter. When the torque converter kicks in (even when driving in 3rd gear) the rpms will drop about 2k).

Easy way to check, put ur car in 3 or kick off ''OD'' & when the car shifts through to third gear u will feel an extra ''shift'' of sorts, & watch for an RPM drop. If this does not happen its the Torque converter. If u r in OD or 4th gear (D), & its RPM is r dropping 1-2, 2-3 & not again, likely 4th gear is out & the tq converter needs to be replaced.

Take it to a reliable transmission shop for a full analysis - Mr. Tranmission or someone in ur town who specialized in transmissions (does not have to be a Lincoln dealer - usually those shops like Mr Trans r a lot cheaper).

EDIT: 4th gear should kick in around 45 under normal driving conditions & the RPM will drop back to around 1800-2000 or so then when the TQ converter kicks in fully it will drop another 1-2k