Good luck with that 2 terminal replacement Fuel Gauge. You should probably be like this guy
and send them back for better replacement gauges. I didn't realize that they made replacements with two terminals until very recently, and already I only hear bad about them.
Anyway, your description indicates the gauges are shorting to ground, the senders are out of spec, or the current in the circuit is out of range for the gauge design.
Search this forum, there are about a billion gauge threads, but I'd start with getting better replacement gauges.