When you have 15.87987652 gallons on board, 15 is less misleading than 16.
Many fuel senders can only send about 20 levels to the EMS. If you look at them closely, they have resistance "blocks" and aren't continuous. Thus, you can't actually get 1/2 gallon accuracy on an RV tank with ~20 gallons. So we only show half gallon increments if you have a tank 10 gallons or less. There isn't a way for you to adjust this.
We know capacitance and higher resolution analog senders exist as well. However, trying to show 15.8 gallons or even 15.5 gallons gives a false sense of security. If the plane is on a slight slope on the ground, your ball is out in flight, or there's a touch of alcohol in your fuel throwing off your capacitive system, or if your tank has a non-linear shape between calibration points, rounding down to whole gallons prevents the pilot from thinking they have more fuel than they do. Which is always a good thing.
In an O-360 running LOP, 1/2 gallon is under 4 minutes. 1/10th a gallon is 46 seconds. Given those timeframes, I don't really see the advantage to trying to show fuel levels to such precision, given the senders and the rest of the system don't have that accuracy.
The fuel computer does show in tenths of a gallon since fuel flow senders easily achieve this resolution.
