Chasing extremely poor MPG...

Hello I've been chasing poor fuel economy for the last year. 08 v6 manual transmission with 209k miles.
Mods: fuel beast wheels, 265/70r17 ko2.
DTLT headers with sim, borla cat back, k&n intake.

For the record, the dive happened long after all of the mods were installed. (3 years roughly) I only ever got 13.5-14.5mpg regularly. However a year ago the fuel economy dropped 11mpg, now I'm at 10mpg. I've had a code for maf and 1 upstream sensor, which was replaced with Denso parts. Mild improvement, but still lower than usual. Since then I've replaced pcv, plugs, injectors, (had new gaskets installed), cleaned throttle body. Checked cats. No codes. Plenty of low end power, but does seem to struggle more on the upper end than it use to. Wheels spin freely, no brakes sticking.

I'm at my wits end. URD pump kit coming in a couple days, but I'm sick of throwing parts at it with no progress. I noticed an improvement running 1 bottle of lucas fuel system cleaner through it. (A whole 11mpg) but right back down after a second. My usual shop has been too busy to do live data, dealership is a joke... i know that's a long read, but does anyone have ANY idea what else might cause this before the last two things are done?

 

I really don’t know if you’ve cleaned the MAF and made sure the tires and inflated to the correct pressure. My last fill up I got 15.8 all city but I drove it like a baby to run a test. Not really sure if that’s any good. What is this Lucas fuel system cleaner stuff? Is it known to help improve more than 1 mpg?

 

What are your fuel trims?

 

02 sensors

 

Look for evidence of an exhaust leak at the headers and pipes, like black soot at a gasket or connection. An exhaust leak would allow fresh air into the exhaust stream and can confuse the O2 sensors, driving the system rich