Reader Question: I just replaced my spark plugs with Bosch Platinum 2. Now I am getting 1 MPG less than I was before the switch.
What is up with that?
Measuring MPG can be tricky since there are many factors that can easily sway your calculation.
Such items can be:
1. Driving habits: You probably do not drive exactly the same way each day, and therefore MPG will vary daily.
2. Weather conditions: Strong winds, changes in outside temperature can affect MPG.
3. Tire pressure: Could you have one or more tires with a slow leak – a huge factor in MPG is tire pressure.
4. Proper engine oil weight: Too thick of engine oil will reduce MPG
5. Air Filter: Dirty air filter will reduce MPG.
6. Types and Grade of Fuel: Different grades and sometimes, different brands of fuel can change MPG.
You might want to double check the spark plug gap just to be sure they are set correctly and one plug gap is not closed to tightly.
I have seen mechanics accidentally hit the spark plug on the exhaust manifold during installation and cause the electrode to bend and close the spark gap.