If you want to improve fuel economy, the best thing you can do, aside from all the basic stuff like a major tuneup and higher tire pressure, is to swap the automatic transmission and 3.73 or 4.10 rear axle for a stick shift and 3.31 rear. This should get you up into the high 20s (low 30s on the highway) with sensible driving, which is pretty darn good for a 3000lb+ car. There are very few conventional (non-hybrid) cars on the market today that can do significantly better than that---even the Smart gets only 34 MPG.

Other minor tweaks that can add 1/2-1 1/2 MPG each are installing a front splash pan, if yours is missing, removing the roof rack (if you have a wagon with a permanent one) and making sure the front end is correctly aligned, with toe at or near minimum spec.

As Rhys pointed out, you won't really gain a thing by trying brute-force "overrides" of sensors in a complex system.

Want better gas mileage? Get rid of excess weight in the car (clean out the trunk), keep the engine tuned and air filter cleaned, pump the tires up to 36 psi, and use the brakes as little as possible. Don't drive right up to the red light; back of the gas and coast there, maybe if you give it a little more time it'll be green when you get there. Do this ten times a day and you'll see you mileage improve.

Every time you step on the brakes you're throwing money away - you paid for the gas to get you up to cruising speed, and it takes relatively little gas to maintain a given speed, but a lot to accelerate to get to that speed.
--
No one ever went broke underestimating the taste of the American public - H.L. Mencken

Hi Steve;
The three major exhaust pollutants ( CO2 isn't one of them....) are hydrocarbons, oxides of nitrogen, and carbon monoxide. A graph of these three on a scale of air fuel ratio shows a small "window" on the stoichiometric (chemically perfect) ratio. NOX opposes HC and CO to produce a valley between them at that point. The purpose of the O2 sensor is to center the average fuel mixture on this point. Too rich and CO and HC rise, too lean, and NOX rises. Exhaust gases in this range can be treated effectively by the catalytic converter, and so the system works in harmony when all is well. This is a long way of saying there is no more "leaness" to be achieved with the engine as it is presently designed.
Fuel efficiency does not always mean lowest emissions, but it's close, and low emissions has to be a major consideration.
Disconnecting the coolant temp sensor will place the fuel system rich; the strategy for most sensors, is to do so to protect the engine during the fault.
A twenty-odd year old engine is ancient technology. I am aware that some feel that driving an old car is somehow environmentally conscious, but it isn't. The most environmentally responsible thing to do is to drive as modern a car as one can afford, probably a Honda or a Toyota, given that their engine management systems are arguably the best on the planet.
If we intend to drive old Volvos well into this century, then our responsibility is to keep them operating as the factory designed them, with all emission systems in place and functional.
Hope this helps,
Rhys