What could be causing my suddenly lower MPG?

Here is some info on rolling resistance and general tire design. I'll let you come to your own conclusions about whether tire weight is a significant factor in fuel economy.

https://www.bridgestonetire.com/learn/tire-technology/tire-rolling-resistance/

https://bndtechsource.wixsite.com/home/rotational-inertia-calculator

https://www.youtube.com/watch?v=0YAoQZXSq4g

https://www.youtube.com/watch?v=cx4YaxEc5PQ

https://www.youtube.com/watch?v=oV_6u4Z5p-c

 

There is no debate. The atmosphere is already 80% N2 and chances are extremely high you had plain ol air in it from day 1 anyway. They just put green caps on it so the salesman could tell you it’s pure nitrogen.

Whatever you do, don’t go looking for more “pure nitrogen” to fill your tires. Good shops will laugh at you with their buddies in back room and just fill them with air, bad shops will say “yes sir that will be 20 bucks” then fill your tires with air anyway.

Being a little pissed at something, anything, is enough to make you subconsciously drive a little faster and lose 3mpg in a flash. You’re good, just drive and enjoy!

 

Unless you are inerting your tires because you are carrying extra fuel in them into a combat zone, there is no need to pump pure nitrogen into them.

https://en.wikipedia.org/wiki/Inerting_system

 

Winter

 

Following up on this, I aired up the tires and it made a big difference. I'm now getting about 22-23 mpg tank average.

 

What was the before and after PSI?