+1 to what Kermy wrote.

Yes, the temperature sending unit (on the engine) can be replaced. Also check the connection to it is clean and tight - the terminal on the sending unit can get lose with age and the connector on the end of the wire can get loose and corroded with heat and age.

As for your smoky centre exit exhaust: Mini exhaust systems can be very difficult to get properly sealed. It does not take a very big leak to have fumes come into the cabin. The second issue may be that has a centre-exit tail pipe. That releases exhaust into a huge low-pressure area behind the car as you drive along. The faster you go, the stronger the low pressure effect and the bigger the volume of exhaust. The car drags this cloud of "smoke" along with it.

By nature, the Mini passenger compartment is under a slight vacuum when the car is moving. This is because there is a low pressure eddy outside each door. The faster you go, the bigger the eddy. This sucks air out of the cabin. It is worse if you open windows. The low pressure in the cabin sucks exhaust back into the car from the cloud the car is dragging around.

A side exit exhaust at least puts the fumes closer to the air stream passing around the car.

You don't say what Mini you have but the one in your profile looks like a MkI Cooper S, so my comment pertains to a MkI. Is your fuel gauge working properly? Both the temperature gauge and the fuel gauge get 10 volts of power from a voltage regulator. If temperature gauge is not working properly because of a bad voltage regulator then your fuel gauge will also not be working properly.

Hey guys,

1 : My Smith temperature gauge stay on 140 to 145 and I know is a bit hotter then that. I just got it running really good. Can the sensor be replace or calibrate the gauge?.

2: I would smell a heavy smoke when driving for a while. I checked all the connection. I just replaced my single to a twin Rc40 center exit box. It smell smokey before but it got a little more.

Thank you for taking time read and any advice.