I got a couple of 20 - 21 mpg tanks in my '99 5 spd right after I got it. But that was an 'indicated' MPG, mostly due to the cute tiny little 245 width tires on it. They spin more times per mile, which racks up more miles on the odometer, more miles = better MPG.
I stuck 32" Michelin AT2's on it next, which dropped the indicated miles down, and MPG's dropped a bit. Then I added weight (armor) and some bigger chunkier tires (33" MFG's) and now MPG's are around 17, 17.5 in mixed driving.
I really think that about all a restrictive air filter could do is limit your peak HP. The fuel/air mixture is very tightly controlled by the ECU as it watches the exhaust via the O2 sensor. Having a restrictive air filter is effectively the same thing as having the throttle slightly less open. No effect at anything less than WOT, and a *very* minor effect at WOT.
Assuming the stock sized air filter isn't completely clogged with crap. And even in that case, it's just more of a drop in peak WOT HP, not a loss in HP. If anything, that will prevent you from using more gas. HP means gas.
In ye olden days of carburetors, sure, a dirty air filter could make a car run rich as hell and get horrible MPG. Because carbs meter fuel into the passing air based on relative air pressures - the air flowing through a venturi creates a partial vacuum, that sucks the fuel through little jets into the air. If the air cleaner is creating a restriction, it creates even more vacuum, the carb in effect thinks more air is going into the engine, more fuel gets sucked in.
In a fuel injected car, even if the MAF/AM sensor was fooled by a slight vacuum from an air filter restriction (and it *isn't*), then the ECU/O2 sensor would catch and correct it.
As for oiled mesh (K&N, et al) style filters, I won't use them on my car anymore. If they are slightly overoiled, they can foul the MAF. The MAF uses a heated wire in the air flow, little droplets of air filter oil land on it, get turned to soot, enough of that going on and the sensor stops working correctly. When I bought my 4Runner the CEL came on and I got them to knock a few hundred bucks off, it ended up being an over-enthusiastically oiled K&N filter.
Even oiled correctly, they can stop working properly if they get overwhelmed with dust. This is absolutely *NOT* a failure mode you'd want from your air filter. Last year's back country Utah and Colorado trip with me in my '99 and my wife in her '96 involved lots of dusty roads. Even following at an appropriate distance, the following car got a decent amount of dust. After a week or so my '99 set a CL and started running poorly, noticeably weaker, harder to start. And a peek in the intake tract showed signs of fine dust all over. A little time with a Q-tip *VARY carefully cleaning the MAF wire and it was back in fine shape again, but I swung by a parts store ASAP and got a paper air filter. If a paper air filter gets overwhelmed, it just clogs up and you lose peak HP. If a K&N style gets overwhelmed, it just start letting dust into the engine. Fouling the MAF wire is a minor inconvenience, the wear created by dust gioong into the cylinders is a bigger concern, if it were to happen more often.
