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Mileage will only be affected by a Car that has a wideband sensor and an ECU(engine management) that has been tuned effectively in the lower throttle range 50% and below for a 14.7 Air to Fuel ratio (aka perfect combustion) of gasoline to air mixture leaving 0 O2 molecules left, aka all CO2 and H2O are left over and no O2. the difference between 91 and 94 is when you get the engine hot and under some load and at a high RPM or under boost when heat is prevalent and you start to have pre ignition or detonation, this is when the piston hasn't reached TDC (top dead center) and the air/gas mixture ignites without a spark, usually on the edges of the combustion chamber pocket where the head meets the top of the cylinder walls. 91 octane will have a lower temperature than 94 octane for pre ignition and thats the only difference between the two. therefore yes in an earlier post 94 is for supercars and 91 is for cars like the mazda 3 with skyactive, yes it still has high compression but mazda built that car for 91 octane, you can't use 87 in the car because on a hot day you might get detonation, but 94 is fine to use, but you won't see many benefits from the gas unless you modify your ECU at a tuning shop on a dynamometer. if your cars stock and doesn't have a rating I'd say use 87, if it has a rating on the tank use that gas, it will never hurt to go up in octane but it can hurt to go down.
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