Gas does not burn any slower on a full tank than it does on a half tank.
A full tank will weight more and would instead if true burn more gasoline than a half tank.
However keeping a full tank is good as it helps keep the fuel pump cool and prevents you from getting moisture in the tank.
But having a full tank or a half tank of gas will not save gas.
Driving the speed limit and not speeding will help you save gasoline.
Good tires and proper tire inflation and proper tuneups will also help improve fuel economy in your vehicle.
A full tank of gas will last longer in that the full tank of gas will go further without having to stop as much to fuel.
However filling up your gasoline tank to full will not get you better fuel mileage.
But it's always a good idea to keep your gasoline tank full when you can to avoid running out of gasoline and to keep the fuel pump cool.
Some ways you can help improve your fuel mileage are.
Get a tuneup, change spark plugs, air filter, spark plug wires.
Inflate your tires to the proper pressures.
Go Easy on the Pedal. Speeding, braking, and rapid acceleration waste gas.
Slow Down. Gas mileage efficiency tends to decrease above 50 miles per hour.
Leave Extras at Home.
Use Cruise Control (When Appropriate)
Turn off the Car.
Check Tire Pressure.
Replace Spark Plugs.
Check the Alignment.
Monitor the traffic ahead and "time" stoplights to maintain momentum and avoid unnecessary acceleration and braking.
Observe speed limits. Fuel economy peaks at around 50 mph on most cars, then drops off as speeds increase.
Reducing highway speeds by 5 to 10 mph can increase fuel economy 7 to 14 percent.