Is it safe to mix premium gas with regular? ?

It's a Mazda 3 - Not a Mazdaspeed 3. You don't need to run Premium fuel, but if you have to because of reasons like the station was out of 87, then you can use it. It won't hurt your engine, nor will it hurt or improve your performance. 

Yes I dont use ethanol so sometimes i have to use 91 octane

well back in the 80's when the gas stations still offered premium fuel without the alcohol in it, I would mix low and hi test fuels to raise the octane.....and had to, since I bought a 67 ford from a co-worker and it still had the factory motor and 94 octane rated too, with just the amoco 92 it'd knock, so I mixed the fuel, per a book dad got me, that said one could mix the octanes, creating  a synergism and it worked..don't think it'll work any longer with the alcohol added fuels.....

If it comes out of a gas pump it is fine JUST AVOID DIESEL. for gas cars and avoid gas for DIESEL CARS. and avoid water for both cars.

Jesus ******* Christ, if it was dangerous it would have been made impossible to do because some moron would do it.

It won't hurt your car one ******* bit.

It won't matter. Use the lowest octane gasoline and don't run higher octane gas thinking it's better if your motor doesn't require it.

Yes it is. Dirty little secret: your gas station already does that to create the "mid-octane" fuel from the other two right at the pump.

Don't waste your money on premium unless you have a high performance car with a higher compression engine that requires it. Yes, you can mix any grade gasoline with any grade gasoline. The antiknock sensor on the cars engine sends signals to the computer to adjust the timing if you change octanes or mix octanes. It does so very rapidly.

OMG NO!!! The last time that happened a rift in time and space opened up in the basement of Joseph Rizzoli's grandmothers house in Queens. They almost lost the cat.

It won't cause an explosion or jelly the fuel, if that's what you mean, but if a car's manual says "use high-octane fuel", you either have to fill it ALL up with 92, or find a mixture that doesn't cause the engine to "knock".