Not a big tranny/differential guy, but 1900 rpm at 60 mph seems a few hundred rpm high. Could the gearing make that big of difference?

even though it was tested good, the O2 sensor can trick ya, on our old 94 we replaced it with an AC Delco, it ran fine for a couple of days and then were having the same mileage problems, went and put in the shop and the guy told me the O2 sensor was showing good reading but was no good cause he replaced it with one from a car he had on the lot and told me drive it a week, my mileage was back to where it was supposed to be so we took back the delco and got another one, no problems up to the day i sold it,

hmmm maybe some stp fuel additive?

What do you drive like? That can be about half of your issue here, but something is probably going on by the look of your highway mileage.

What plug brand and heat range are you running?
X2 on the 02, it could be bad out of the box.
x2 on the fuel injector cleaner. Have you ever checked the tps voltage at idle position?
When was the last fuel filter change?

im getting 18MPG with about 70-80% city.

I am using stock Delco plugs put in about 6K ago when the motor was built. Fuel filter was done last year when I bought car. I run at least 89 Sunoco. I have ran a BG Fuel system treatment including a injector cleaning through the fuel rail. My driving is a little on the heacy footed. But, I did some "normal" driving and the mileage wasnt much different.