Own 2006 Volvo S40 just replaced reservoir tank and added 1 Gallon antifreeze still running hot
Don't know why?

 

The fact that you replaced the reservoir tank and added a full gallon of coolant suggests that the car overheated due to lack of coolant the first time. This first overheat probably blew the head gasket.

Now it will consume coolant. You have to fix the problem.

 

did the car overheat at some point? (ie with boilover that damaged the overflow reservoir) If so you may have a thermostat issue, radiator issue, water pump or as tony1963 noted you may have a head gasket issue. Best to get it into a shop right way as you can further damage the head with a more serious overheat if in fact its a head gasket issue.