Improve mobile usability by specifying number input fields
Use type="number" on the milage and gallons HTML input fields.
Using type="number" instead of type="text" in the input form elements would make (some) smartphone browsers automatically bring up a numerical keyboard instead of the default qwerty keyboard when entering these numeric values. This should be a tiny tweak and would result in a very noticeable improvement to mobile usability. Here is a good overview of various such options available in HTML: