I have an '89 volvo 240 that I've put about 150K on myself, the clock is now around 340K miles. The drivetrain is essentially the same as yours and I doubt our computers are very different. Mine is LH2.4, if that tells you anything.
Mine had a belt-driven radiator fan that I replaced with an electric one harvested from a Volvo 960 (I-6 engine) with a blown timing belt and thus wrecked valves. Our 2.3 liter four's are non-interference engines. That was a very worthwhile project. Even when the fan clutch isn't engaged, the belt drives the fan and it presents a load to the engine. That load is gone with the e-fan installed.
To mimic oem engine cooling as well as possible, my e-fan is switched on via two methods:
1) Brake switch. If I hit the brakes, it comes on. I did put in a relay with a delay so it doesn't come on for a few seconds, and stays on for a few seconds once activated. This reduces fan cycling. Typical rad fan switching doesn't include this switch but as I noted above, the oem belt setup drives the fan always, even if the engine isn't particularly hot. So I thought it wise to provide airflow when stopped by this method. This avoids having the engine get warmer than usual from traffic light idling etc.
2) Temp switch in the lower radiator hose. I tried switches of different temp values in upper and lower hoses and got best results in the lower hose. The switch's temp setting is something like 165? or 175? F because the cooled water has to be noticeably below the operating temperature in order to cool the engine. Sorry I don't recall exactly what temp value switch I used though.
Currently getting +/- 50 mpg in fall weather. EPA is 31/39 so not too shabby. WAI, fuel cutoff switch, full belly pan, smooth wheel covers.
I have a 88 740 turbo that I managed to get 27mpg out of driving something like 85-90% freeway. Never driven it on a roadtrip/100% freeway driving.
31mpg is excellent to me. I wish I could get 30mpg. Now that my OD unit just failied I'm getting 20. I just filled up and it looks like I might get 25mpg with driving 60 or lower, etc.
I've done the basics; tune up, fuel filter, air filter, tb cleaning, pcv service, etc. I need to check and see if my air inlet t-stat is working correctly. with 150k the o2 is still working..... I was thinking about pulling all the a/c parts and converting the fan to electric. but I don't really see it making a difference. I did inflate the tire to max, but caught a nail in a tire and didn't notice it so the results where botched.
interesting info is that in the owners manual it says 30psi F and 36psi R gets the best mileage. and shifting early and using more throttle gets the best mileage. I got my shift light working for a day and it wants you to shift at 2.4k to 2nd and 3rd and like 2.1 2.2k to 4th.