I havn't change my plugs, wireset, oxygen sensors, air filter or PCV valve since I've had the car. I don't think they've been changed before I got the car either. I'm planning on changing all these parts when the weather gets a little nicer out, but I also wanted to know: when I change these parts in my '95 ETC, am I going to get better fuel economy and if I do, how much more MPGs is it roughly going to give me ?
Thanks

 

if the air filter is clogged now, maybe another 1 mpg,
the rest, zero.

the criteria you gave here doesnt help to understand why you are doing this.

Sure, I would change the PCV and the air filter .. cant hurt.

 

Then how is a tune-up supposed to give your car better fuel economy ?

 

By bringing the engine BACK to specs .. if the engine is running to specs, there is no gain to be had.
Why are you considering this?
car mileage?
bad performance?
engine misfiring?
fault codes?
dropping fuel economy?

just not enough details ...

Air and fuel filters, big YES,
PCV valve if you have one? yes
coolant change out? yes
brake system flush? yes

 

Back in the olden days most ignition systems had points which wore retarding timing, ignition coil output was low compare now, gas wasn't that great, fuel injection was primitive - no feedback. As Stealth stated tune ups were more involved to get the performance you wanted MPG or HP. The 100k plug replacement interval shows spark plug replacement isn't as important as it once was. Things that can help fuel economy : driving habits, wheel alignment, tire pressure, clean air filter, clean induction system(no carbon build up), proper operation of brake system (not dragging ). All can add MPGs, but they will never turn an ETC in to a Honda in terms of fuel economy.