[dongs]

I'm reasonably new to ecomodding/hypermiling and a week and a half ago I took the first step towards improving my FE by installing a vacuum gauge. I'm pretty limited income wise so a scangauge is out of the question for the time being.
About 90% of my driving is commuting to and from work, the entirety of that is on a 2 lane 35 mph road/hwy through a couple stoplight infested towns and rolling hills. I've got a 2002 S10 with the 2.2 motor and a 5 speed and I had been averaging 22-23mpg or so before installing my vacuum gauge, with my last tank at 22.8. I also rotated my tires and while I was at it upped the pressure a bit (they were a bit low now they're at 40 psi, manufacturer recommends 35, sidewall is 45).
Well, I filled my tank today for the first time since adding the gauge and my tank mpg was 23.58, an increase of .78mpg or about 3.5%. I guess I was expecting a slightly larger increase, especially since my driving habits on my route have since changed quite a bit. I realize calculating tank to tank mpg by dividing mileage by fuel added isn't the most accurate, and in Oregon you aren't allowed to pump your own gas(in fact when I filled up today and did my calculation I figured 27.2 mpg and about sh!t my pants until I started my engine and realized the goober pumping my gas was about 1/8tank short of full. Then I don't know if he 'topped it off' when i asked for him to pump it again since it wasn't full)
Anyhow, is 3.5% a reasonable increase?
I've been thinking for a while about replacing my upstream o2 sensor since my truck is 11 years old with 131,000 mi. Do you all think that would be a reasonable investment?
It's about time to change my oil and I'm thinking while I'm at it I'll swap the trans & diff fluid to synth. I've run mobil1 in my motor before but it sure was quick to point out a leak in my rear main. I've been running conventional HM since then with no leaks to speak of but I may switch to mobil1 HM to see if that helps at all