I'm guessing
you have an outboard, if so the MPH runs from water pressure around the foot.
If you have two fuel tanks the you should have two seperate fuel guages. When he changed the tanks he may not have correctly hooked up the twin tanks correctly. To get them to indicate correctly you would need to run two guages or you could try to set up a system like in p-up trucks that have two tanks (switching valve that swap fuel lines and controls two sending units).