slowly decreasing fuel economy?

[JHahn22@xxxxxxx]

Hello again folks,
Can anyone give me some tips on how to easily figure out what is going bad on 
my 89 325i?  When I first got it (at about 92k mi.) about 5 months ago it was 
giving me nice 26.5 mpg on a long highway trip.  The last long highway trip I 
took about two months ago it was at 23 mpg.  I deliver pizza in it, and when 
I was first delivering, it was at about 18-19mpg around town.  Now it's 
around 14.5.  I replaced the spark plugs which were corroded, and it may have 
helped a little, but not much.  Perhaps I'm learning to drive in a more 
"spirited" manner as I become more "one with the car".  But I think there are 
things that need fixing.  I checked the lash a while back and it seemed ok.  
I wouldn't be surprised if I need to replace the O2 sensor, wires, cap, 
and/or rotor. . .but all those things cost $ that I'd rather not spend if 
that isn't the problem.  I have a multimeter, but have not been able to 
figure out what all those functions are and what all I can do with this 
gadget.  Since I plan on driving this car until the wheels fall off, and then 
buying more wheels for it, I'm thinking one of those Peake Research engine 
diagnostic thingies might be worth the $ rather than paying a shop to do it.  
But since the check engine light hasn't come on, am I likely to gain any 
useful info from one of these?  Any tips would be greatly appreciated.  
The more I drive this thing, the less sure I am that I want to buy a newer 
car when I'm done college.
Thanks for all the help,
Jake Hahn
89 325i(s)
87 Alfa Romeo spider

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