Using just the 4R's built-in Average MPG gauge, I came in at about 21.5 by the time I stopped driving. In ideal environmental, traffic, etc. conditions, I can see where someone might reach 23.
The factory MPG gauge?

ound:
You mean the "Lie-O-Meter?"
Or the "Wish-o-meter?" (as in, "I
wish I got this kind of MPG?")
From what I've observed, that meter typically reads ~ 2 mpg high on a regular basis (when compared to hand-calculating.) So your 21.5 was most likely 19.5 or so - respectable for a V8, for sure, but not out of the realm of possibility.
I've had that meter show me getting 25 mpg before.

No, it wasn't 25, hand-calculated it came out to 21 - 22, but that was under absolutely ideal conditions (gentle downhill driving from Laramie, WY to Denver, speeds at 55 - 60 mph.)
Actually, there is one "gadget" that the OP could buy that would help at least a little: Something that shows real-time MPG can help you adjust your driving so as to maximize MPG. I've used both the Ultragauge (UG) and the Scangauge II (SGII.)
Currently I run an SGII on the 4runner because it will monitor transmission temps, which the UG does not. I have the UG on my DD Mazda B2300 pickup (A/K/A Ford Ranger) which has a manual transmission. Both have helped me get better MPG by showing me exactly what kind of driving causes my MPG to plummet and what allows it to soar.
Whether the SGII or UG will save you enough money to justify the cost is, IMO, irrelevant because they are useful diagnostic tools to show you what is going on with your vehicle, so they serve other purposes as well, the MPG monitoring is just icing on the cake. They can also show you diagnostic codes and even reset your CEL (useful for me since the Mazda has been throwing a code for the last year.)
The best thing about the aftermarket gauges when compared to the factory gauge is that you can calibrate the aftermarket gauges to take into account bigger wheels, etc. My 3rd gen had the notoriously unreliable fuel gauge so I used the SGII as my default fuel gauge, since it can show DTE - Distance To Empty.
I think I wrote up a comparison between the UG and the SGII a couple of years ago, I'll see if I can find it.