The gauges and the senders are designed to operate together. Your Land Rover uses a 30-300 ohm sender (30 ohms=cold, 300 ohms=hot). Your aftermarket gauge apparently doesn't.
You need to install a sender with an ohm range matching the specifications of your particular sender.
Which sender did you get at NAPA? It should be easy enough to tell what the correct sender range for that gauge is.
--Mark
1973 SIII 109 RHD 2.5NA Diesel
0-54mph in just under 11.5 minutes
(9.7 minutes now that she's a 3-door).