I was telling my buddy, who's a mechanic, how my fuel MPG increased since I put in a new thermostat. He said it would be even better if I put in new plugs, air and fuel filters.

Since she runs perfect now, I'm afraid that changing the plugs might screw something up. Would it run even better with new plugs and filters?

 

Depends on the condition of the plugs and filters. If they are are pretty far gone then you will notice a difference.
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+1

 

If you put OEM plugs wires and filters in, I will guarantee you will not get worse mileage. But like said above if they are in good shape it will make no difference. If you have more that 50K miles or five years since last tuneup, I would replace them. Do the distributor cap and rotor also. It's pretty easy to do on these cars. You can probably buy plugs, cap, rotor and wires for around $125. Even less if it's non-turbo. The OEM Turbo plugs are around $32.00. Use OEM wires and plugs.

 

If the last time the car was tuned up is not known, I would do it.