If your vehicle calls for 91 octane gasoline and you put 87 octane gasoline in the vehicle instead then you will have a decrease in mpg and have spark knock which sounds light a rattling noise or high pitched pinging noise and reduced acceleration.
In some cases the lower octane gasoline may cause damage to your vehicles engine.
If you mix premium and regular gas in a vehicle then the combined premium and regular gas will result in an octane level that is around somewhere in the middle.
The vehicle will be okay on the premium and regular gasoline mixture and no harm will be done to the vehicle.
If you put 87 or even 91 Octane gasoline in a 93 octane car then the engine could begin knocking and could also be damaged and you would void the vehicles warranty if you have a warranty on it.
If your car calls for 93 octane gasoline then you should use 93 octane gasoline when possible.
93 Octane gasoline does not increase MPG.
Unless your vehicle calls for 93 Octane gasoline then it's not worth it to buy the more expensive premium gasoline.
The cheaper gasoline will give you the same MPG as the premium gasoline and your engine will also be just fine.
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.