My 98, 3.9, 4x4 5 speed is averaging 13.5 mpg mixed highway and country roads. I have replaced the distributor cap, rotor, and ignition coil, new plug wires, Autolite 3923 plugs, new injectors, new TPS, new idle control valve, new injectors, cleaned out the throttle body, 2.5" exhaust with high flow muffler and an air intake. Is this mileage normal or should It be better? The truck runs great and doesn't have any codes.

 

How old is the O2 sensor?

 

Yep, I would look at your o2's and if you have a scanner that reads live data, look at your fuel trims to see what they are doing.

 

I have a scanner what should the numbers be?

 

long term should be pretty close to zero.

 

13.5 out of the V6 is about right for mixed driving. I would expect closer to 14 out of a 5 speed, but I would't say you're far off.

 

I was able to get 12 to 14 out of my 96 2500 with the 360 around town...... I would hope that a lighter truck with a smaller motor would be able to do better than that...... But, I have been wrong before.

 

I never got better than 12.5 out of my V6 (mixed use) until I went to 4.10 gears fro 3.55, then it came up to 13 mixed.

The issue with the 3.9 is that it doesn't have enough torque to get moving down low, and takes more throttle to keep the same speed. It's just generally grossly underpowered. That's why 3.55 to 4.10 picked up my mileage. a 4x4 should already be 3.92 if i'm not mistaken.

The over head likes to show 1-1.5 happy from actual.

Now that I have the powerglide and a custom SCT tune from Ryan at www.frptuning.com, I'm still averaging 15 or so on the highway spinning 3000-3500 RPM.

 

It always sorta bothers me seeing other people's mileage with the 3.9... I have a 98 RC 4x2, so it's lighter of course... but does that weight make that much of a difference? The LOWEST I've gotten was in winter (which lowers anyway because of the mixture change) before I actually did tuneups and whatnot when I got the truck, was 15-16mpg (burned O2s and lame 3rd party crank sensor didn't help). I generally have mixed 60/40 city/highway and my average is 17.5-19mpg, pure highway I get 20-22mpg. This is with a 42RE automatic, 3.55 Rear diff, and 235/75/R15 tires, I would assume a manual would manage better. Pretty much stock everything as well, just a short ram intake and magnaflow cat from the previous owner.

 

I have 3.55 gears in mine but I plan on installing 4.10s in the future, and possibly doing a 5.2 swap.. I looked at custom tuning but most of the tunes require premium gas, which defeats the purpose of better fuel mileage, because premium costs 40-50 cents more here.