1992 cadillac deville looses power over 55 mph.

since it is consuming oil,time for overhaul.if it was alighnment then it would start at 20-30mph

If it has a gas line filter, replace it might help..

try the fuel filter first. check air filter too.

May be the fuel lines or bad distributor

There is a damper (basically a door) in the heater unit that is probably not being actuated. This damper allows the air from the blower to be sent over the hot heater core and thus warming the car interior. This damper might be either vacuum controlled or electrically controlled. My 1992 Blazer had vacuum lines that went to it. It could be that you have the vacuum lines loose under the dash. Look for loose hoses or connectors and see if you see an obvious place they might plug into.

Check your fuel filter for restrictions-Fuel filters are often times overlooked

a pressure test will tell for sure

Check the air filter for a blockage. If you have a scanner, check to see that the TPS (throttle position sensor) is going all the way from 0% to 100% opening as you press the accelerator down all the way, while the ignition is on and the engine is not running.
Check for codes that might lead to a solution, if the above doesn't help.

Is it cranking? or do you hear clicking sounds?, It could be loose battery conections. A loose connection could stop your car from starting. Was your car running fine before you replaced the battery?

This could be a transmission issue or a catalytic converter issue hard to say with the limited info provided