Let me start off with - I still need to change the spark plugs. They aren't horrible, but have been in this thing for a couple of years (I bought it in February of this year).

things that matter:
2000 4dr 4wd Jimmy
162,000mi
1.5" crank/shackles
31x10.50 Firestone ATs
K&N drop-in with opened up air box
Dynomax cat-back
accel super coil
!cat

truck runs pretty good. The power doesn't seem to be as good as my 1999 Blazer had (w/o any mods). Mileage on my blazer was 200,000 when it left.

I get between 200-250mi on a tank. I usually run it just under 1/4 tank before I refill. Last tank got 17.0 mpg. I'd like a little more. I was averaging about 18.5 on my blazer.

I admit up front that changing the plugs could help some.

Am I expecting too much from this 4.3 in a 5,300lb envelope...?

My TBI 4.3 in my old 94 S-10 ext-cab 2wd with many bolt ons (turbo city stuff, edelbrock headers/intake manifold, and more) got 22-23 in regular driving and could hit 25-28 on the highway. It also weighed about 1,500lbs less than the Jimmy.

 

The extra weight certainly accounts for a big loss of MPG. This vehicle weighs more and has less HP than your s10 did, thus it takes more energy for the Jimmy to move. Most people with the 4.3s used in the first gens say they get better mileage. HP to weight ratio is definitely a considerable factor. You could get the PCM tuned from PCM4Less or Wait4Me or get a Hypertech. I do say though, I'm extremely jealous of your mileage. I have a CAI and Hypertech programmer and I'm getting 16.5-17 consistently with most of my driving being city.

Do you watch your RPMs? I used to just accelerate without noticing that, just kinda went about my business but never floored it. this was before my CAI and Hypertech, and just from watching the RPMs I went from 14.xMPG to 15.7 consistently by keeping the RPMs under 2500 in first gear, and below 2000 in all other gears.

 

being a gearhead from birth.......the only thing I can keep the rpm below 2500 on is my diesel tractor. Any vehicle I have will see at least one near to or full-throttle shift per day. I think this should be law. If the government dorks would allow BP and contractors to skim the oil off the gulf we'd have lots more fuel to burn and the price would go down.

That said....I don't like re-fueling so often. Wife's 1500 4wd suburban gets 19mpg with a 5.3 hauling that 7,000lbs around. 100hp difference and even more torque.

 

what do you mean opened up air box. do you mean you cut the bottom out of it or what because i bought my blazer with the bottom cut out and it didn't seem to have any power till bought a new bottom