Microscopic issue that causes a big difference

[Blk fiesta st]

Hello people of the forum world, coming to you guys yet again for help because yall are probably some of the smartest when it comes the the fiesta st. So I recently have been experiencing a huge loss of gas, very noticeable. 100 miles from work to my house to my gals house, about a 20 mile drive total and I try to stay in a reasonable rpm to get good gas mileage but it just doesn’t seem to want to work with me, also experiencing a slow loss of boost as the car warms up and has been driving for a while, tuned for 27. Will hit that mark then after about a couple miles boost only will hold at 25. About to go and fiddle with my intercooler pipes check for any damaged hoses but I have been at this for weeks. The gas gauge also seems to flex different numbers at a stand still rapidly, so these problems correspond with one another?

 

[Intuit]

Re mileage, check for codes.

Re fuel gauge, temporarily disconnect ALL aftermarket accessories/changes/additions including something as trivial as LED light bulbs. Check your ground points, including the battery. Search forum as I've seen at least a couple of fuel gauge posts though I don't know that they were actually resolved.

Research what "voltage drop testing" is and how to use it. Valuable knowledge for locating bad connections. Straight pin can pierce wire insulation to get readings. ALL circuits must be loaded to their potential for the tests to be effective. (so that means running all lights, rear defrost, blower on high, elevated engine RPM, etc) Found out from personal experience that aftermarket accessories can effectively "flood" the ground with excess current or noise, messing with functions of the various computers in the vehicle. Because the spikes in current are so brief, voltage drop testing may not necessarily work for that scenario. Only an oscilloscope would.