Drastic decrease in MPG
 

About 2 weeks ago, I sold my truck and was looking for a manual trans mustang. I owned one previously before I got a truck, so I knew what to look for in one. I found an 05 manual with 100k miles in perfect condition. The gas pedal when test driving was extremely reactive and sensitive. When I first bought it, it was getting 21 mpg. Since then it has drastically gone down to 16.5 and is still dropping. I don't drive it like a bat out of hell, I baby the thing and the gas pedal. Also, it seems as if I have to push the gas pedal more to get it going and enough RPM to start releasing the clutch. Not as sensitive as I first remember. (Might be because I was used to driving an old truck). But still. I push the pedal and nothing and then it jumps up to 2,000 rpm... I'm curious as to why these 2 things are happening. I do live in a small town and the speed limit doesn't exceed 45 mph and hardly go out of the town. And start and stop a lot. But I feel as if it shouldn't effect it by 5 mpg in 2 weeks. I wonder if these 2 problems go hand in hand. I tried looking up potential issues, but couldn't find anything other than air filter and o2 sensor. I did clean out air filter, but nothing changed. Could it be an o2 problem?

Welcome to the site..

How long had you been driving/checking mileage to prove out the 20ish MPG's under your conditions?

20+ sounds high for stop n go city driving if ambient temps are higher now, along with Density altitude changes if it applies , terrain, stop n go, loading are all going to affect mileage. But since you had the seat of your pants feel for its performance, your the best judge. Any blended fuels used?

The throttle lag for a fly by wire throttle isn't unheard of..
Most anything that can greatly affect MPG, lean/rich, non spec fuel trims, unmetered vacuum or fuel bleeds can affect trims, 02/ monitors etc but with a significant MPG drop generally will trip a CEL.

Might try a throttle relearn/reset..

Turn ignition ON, wait for warning lamps to prove/cancel but don't start the engine

Press the throttle all the way down in one smooth motion at a normal throttle rate of 2-3 seconds until it bottoms out. Let UP as soon as pedal travel bottoms out

Turn the ignition OFF and wait 3-5 seconds

Start the engine normally and test drive for any improvement
If no improvement, repeat a few times and re-evaluate.