Ultra gauge and twelve MPG

If I buy one of these will it help me diagnose why my MPG is so terrible or am I throwing away $120? I'm calculating MPG by gas gauge and odometer, I know that's not accurate but it's ballpark.
2007 Double cab 4x4 77K
I have:
Change oil every 3-4K
Changed plugs 1k ago
Cleaned throttle body
Changed air filter
Changed front and rear diff fluid
Stock size tires @ 34 psi cooper a/t

The truck runs fine although slightly lacking in power, you can't even feel it idle. No engine light.
The shop down the street claims to have dropped the trans pan and changed fluid 5 months ago but did they really drop the pan or just drain from the plug and add. I think the latter.

I think maybe a cat sensor, plugged cat, but wouldn't that throw a code?
At my wits end, can't afford another rig.

 

Have you thought about taking it to a good mechanic?

The ultragauge can help you diagnosis or troubleshoot the issue because it can display fuel trim, etc and other data points

But if you can't interpret the data to use for troubleshooting then you'll probably think its a waste of money.

If you're not supercharged and only getting 12mpg... you're dumping fuel. Check for vacuum leaks first. 18yo rubber lines can become brittle/cracked.

 

Cleaned MAF sensor? That has affected my performance and MPG's in the past.
My Scangauge primarily helps me modulate my gas pedal to get better mileage.

 

December? If you're in the Rust Belt this time of year your fuel mileage can take a big hit. One time during a week of sub-zero weather and short trips I saw 9.3 mpg and my Tacoma is basically stock. When I say short trips, I mean sub 5 miles.

 

Boom. I've watched my mileage take a serious dive since winter blend fuels. I mean, like a major drop; 3-4 MPG less than summer. I look at my Scan Gauge and often see 14-15 MPG averages on the same daily commute. Summer, I see 18-20.

 

Bada Boom. Became very noticeable the last month. I wonder if non ethanol fuel has a winter blend.