I am going to try asking this again. Not sure why I didn't get any responses the first time but here goes again.

About 6 months ago I did a complete brake job on my 2011 Mazda 3S. Pads, rotors, and fluid swap/flush. Also, at the same time I had my cooling system flushed and filled as well. Almost immediately I noticed about a 3 mpg drop in my gas milage. For the life of me I cannot image why either brake job or coolant change would cause that but can anyone think of why that might have happened?

 

NO DTC's reported.
If you are not hearing any bad noises from your brakes or an unusual wear pattern then it is a coincidence . The work you did really could not effect MPG without you noticing something irregular with the brake wear.
I am not saying it did because I can not physically test your MAZDA and check the fuel you use....
Something to look at??
https://www.eia.gov/todayinenergy/detail.php?id=11031


I will mention that in the last couple weeks here in CA in the shop we have had a few customers complain of loss of MPG that they normally get.

 

Nothing other than the occasional brake squeal. That is what I thought but a drop of 3-4 mpg is big drop in that short a time span. Just can't think what else it could be.

 

Ok well if you want pull any spark plug and take a few good pictures and post it. Let the engine run and drive it for about 20 minutes or more, then try not to let it idle to long in other words shut it off as soon as you can when you park. The plug will not be a true reading (most all the info on the internet and well over 50 years old on how to read plugs) but will at least give a starting point about what is going on. We can have a look at them and then maybe a power pull and inspect later and the same spark plug. I will explain if needed.
But 3-4 this time of the year is not really uncommon. Most dont even notice it.