Fuel Tank -16 gallons.....why can I only put in 10 gallons?

And I'll add this from a knowledgeable service tech at Auto Meter (with regards to aftermarket senders): ...(Auto Meter) gauges are calibrated to match a factory sender, which is very non-linear. 10 ohms = Full, 25 ohms = 1/2, 50 ohms = Empty, and 73 ohms = below Empty. With this said, most after market, or reproduction senders are built linear which means that E & F will be correct, but everything in between will be skewed.

I know this doesn't apply your issue directly but it's useful information.