Matt, Several other folks above have mentioned that "topping off" in-cab mounted tanks to the point it leaves little to no room for expansion will result in the gas overflow problem you described. The fill tubes attached to these tanks leave very little room for expansion to begin with and yes, gas does expand A LOT when it's temp increases between in-ground cooler storage tanks to sitting in a hot sun enclosed cab and tank.
Using the coefficient for gasoline expansion 0.00053 per degree Fahrenheit, a 10° rise in temperature equates to 10.9 ounces. A 30° rise would equate to over a quart!
EDIT: Adding to this equation, the average ground temperture at 5’ deep is 55. On a 90° day the cab sitting in the sun could easily go over 130° or more. 130°-55°=75°. The would be well over a half of gallon (0.636 gallon).
Reference:
BSI Engineering
