Different spark plugs and different MPG!

I just made a small discovery. On my 2005 I pulled out the existing Denso iridiums at 135,000 miles and put in a set of NGK iridiums that have 80,000 on them from a spare engine I have in the shop. My mileage dropped a solid 5 MPG. I was getting a pretty dependable 50 which dropped to 45 after the plug change.

Then, putting snow tires on dropped it down to 40 MPG.

I also noticed once when I put in a slightly heavier viscosity oil that this knocked the mileage down by 3 or 4 MPG.

I also got over 70 MPG for a two days while driving with a strong tailwind from CA to AZ.

This is simply offered FYI.

These cars are very sensitive.