Fuel mileage drop
4 Answers
Did you check the oil level? What weight oil was installed?
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What Engine in this vehicle? Mileage? Any warning lights on or flashing when the engine is running?
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If your oil had been really low before the oil change that could explain it, less oil causing drag on the crankshaft will rob less power
And check your tire pressures, it's highly unlikely that after a month you'd see any adverse affects of an oil change, that's something you'd notice very quickly