worst mpg I've ever heard of

Something doesnt sound right. 31" tires arent that big. Even with that weight it shouldnt feel like your pulling a huge trailer. You said the cats were plugged. Are you sure they are clear? A constricted exhaust will make it loose most of your power and consume lots of fuel. Secondly, just gutting the cats could put the system into a loop mode which will make the truck run lean and use more fuel.

 

The weight of the wheels and tires greatly effect power needed to start off from a stop. Size isn't always meaningful, but larger tires weigh more. As does load size. I am running stock tires and wheels and have added a cap and sliders so not much extra weight. I can get 20mpg, but I drive highway all the time. Much like others reported, your friends are fibbing a bit. However, not many of us drive these for mpg ;-)

 

Yeah true don't get me wrong I'm not expecting Prius mileage lol. I just want to get to something practical say 15 or above.

 

Post pics.. Still.. Again.

 

This is your problem. The fuel / emissions systems use the feedback provided by the O2 sensors for proper operation. The O2 Sensors operate by sensing the exhaust gasses. You have basically gutted the system and screwed the injector pulse width calculation all to hell.

Instead of back yard engineering maybe you should fix it instead of just beating on it hoping something will improve.

Read This:

PCM Basics 101:

The PCM controls the fuel injectors by using signals from the engine sensors, like MAP, MAF, RPM, ECT, and others to pick out the right value from the right lookup table to use in the fuel injector pulsewidth calculation. The PCM injector pulsewidth calculation also uses the INT and BLM correction factors based on O2 sensor feedback when it is in closed loop. O2 sensor correction is not used in open loop.

Note that there are 2 kinds of open loop. Right after a cold or hot start, a scan tool will report that the PCM is using open loop fuel control. This version of open loop is described in the next section. Once the PCM decides that conditions are right, it switches to closed loop fuel control, which is described below the open loop discussion. But even after this switch, if you lean on the throttle hard enough, the PCM will switch to power enrichment (PE) mode, which is another form of open loop. In PE mode the scan tool may report that the PCM is in closed loop, but it should also report that learning is disabled. Learning disabled means that the PCM is ignoring the O2 sensors, which is the same as saying that the PCM is using a version of open loop fuel control.


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Open loop

When you fire the beast up, whether the engine is stone cold or fully warmed up, the PCM starts out in open loop. This is because cold (or just warm) O2 sensors don't work very well, and they take a little while to get good and hot. Most LT1 O2 sensors have built-in heaters to speed up the process, which is why they have 4 wires coming out of them. Also, cold engines are happier when they are run richer than the fixed closed loop air/fuel ratio target. Since the O2 sensor signal is not reliable right after the engine is started, the PCM has no way to find out whether the engine is running too rich, too lean, or about right. But it still needs to take a stab at the right injector pulsewidth.

When the engine is running in open loop at light to moderate loads (idling, city driving, highway cruising), the PCM uses the AFR table for the pulsewidth calculation. The AFR table uses the ECT (colder engines need to be richer than warm engines) and MAP (higher loads require a richer mix than light loads) sensor inputs to point to the desired AFR for those temperature and load conditions. As always, the MAF sensor tells the PCM how much air the engine is inhaling. The PCM knows how big the injectors are (the amount of fuel is delivered per millisecond of pulsewidth), so with the MAF sensor output (the amount of air inhaled by the engine) and the AFR from the table (the desired air/fuel ratio), it can estimate its best-guess injector pulsewidth. Remember that the PCM has no idea whether or not this guess-timated pulsewidth is anywhere near right. It could be causing the engine to be very rich, or very lean, or anywhere in between.

Remember that engines like a richer air/fuel mixture when they are cold (richer than the fixed 14.7:1 ratio used in closed loop), and when they are asked to make more than just a cruising level of power. So the AFR table values for high MAP (heavy load) and low ECT (cold engine) are lower (richer) than the values for low MAP and normal operating ECT. This means that "cold engine enrichment" and a bit of "power enrichment" are built right in to the table. This quasi-power enrichment may be used in open loop when you maintain your freeway speed up a steep hill in high gear.

When is the engine ready to switch from open loop to closed loop? The current theory is that the PCM decides to switch based on some combination of ECT and engine running time (3 minutes?).


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Closed loop

When the engine temperature and time-since-the-engine-started timer agree that the O2 sensors are warmed up and ready, the PCM switches to closed loop (at least this how I think it works). Just as it did for open loop, the PCM calculates the closed loop base pulsewidth using numbers programmed in lookup tables.

While the target open loop air/fuel ratios are contained in the AFR table, the closed loop air/fuel ratio is hard-coded in the PCM to be 14.7:1. The MAF sensor tell the PCM how much air the engine is inhaling, the injector constant and injector-offset vs. voltage values tell the PCM how much fuel the injector will flow for any particular pulsewidth, so the PCM just does the math to calculate the appropriate injector pulsewidth.
Closed loop operation may refine the injector pulsewidth calculation using the 2-dimensional VE tables, which use MAP and RPM as the 2 inputs. In the 94-95 f-body (OBD-I) PCM there are 3 different VE tables, one for cranking/starting (0-340 rpm), one for low rpm (400-2000 rpm), and the third for high rpm (2000-7000 rpm). If used, these tables would account for the fact that an engine does not necessarily use all the air it inhales for combustion. For example, engines with long duration cams pass some of the inhaled air right out the exhaust valve during overlap.

Even in closed loop, the injector pulsewidth calculation is really just a guess based on the MAF sensor, the injector constant and offset, and (possibly) VE table numbers. If the pulsewidth is not right, the fuel/air mixture will be too rich or too lean. It's very important to remember that the PCM tries to keep the AFR at the "ideal" ratio of 14.7 while it's in closed loop.

This is where the O2 sensors come in. They generate an electrical signal for the PCM that represents the amount of oxygen in the exhaust stream, which is proportional to the air/fuel ratio. This signal, which is the feedback that "closes the loop", tells the PCM how good a job it's doing on the fuel mixture. The PCM uses this feedback to calculate a correction number, sometimes called the integrator, or INT for short, or (more accurately IMHO) the short term fuel offset. The PCM uses the short term fuel offset to adjust the pulsewidth calculation so that the air/fuel ratio homes in on 14.7:1.

You need the cats and the O2 Sensors to work properly to get the correct fuel air ratio.

 

are you running mud tread tires, op?

 

Bumpers are not your friend. I get 12.5 mpg regardless of what I do.

 

I have full skids, plate bumpers F & R (w/ tire swingout on rear bumper), a bed rack and a bed full of junk. 33" skinny tires. I swapped to 4.56 gears in both axles and 17mpg is all I can get if I really keep the speed down on the freeway. 4.0 L V6, auto trans here. Swapping from 3.73 to 4.56 gears did not hurt the mileage much, but now my trans does not "hunt" when climbing the slightest grade. But 2,000 rpm is like 55 mph now, so no way to "Keep it under 2000 RPM" on any kind of highway. I do not use it as daily driver unless my true DD is down like it has been the past couple weeks. All I can say is I am glad gas is cheap right now!

 

Maybe time to do a full tune-up, replace O2 sensors and clean your mass airflow sensor?

 

E brake dragging?

 

As to getting 20MPG: I had a 2WD first gen Tacoma with the good old 2.4L 4 cylinder and 5-speed manual. It would get 20 MPG if driven easy on the highway, best I ever saw was 22MPG and it had the skinny car type tires and only a bit of luggage for load. The Toyota V6 has never been known for good MPG, either the 3.4L or the 4.0L. The NEW 3.5L V6 is allegedly pretty good, but I'm not spending $40k to find out.

 

To add onto that..
I got 23/24 MPG when my truck was stock, small highway tires, an automatic trans and a 4 cylinder.

There is no way anybody is getting that with a lifted V6 on 35s in any kind of driving conditions.

 

I can also say with my 35's NO WAY IN HELL HIGH TEENS I am pretty much stock and I get 15mpg average

 

I am also re geared <_<

 

I've got a 06, steel front and rear, sliders, ifs and trans skid, automatic and I get about 15 if I'm nice to the skinny pedal

And NOT re-geared... Yet

 

I am Stock DCLB Automatic with 265/75R16 Duratracs (C Load) and ARE CX Canopy and have been getting 19.2mpg and that is using actual distance traveled with a GPS factor since the larger tires you can not use the odometer as an accurate readout.

With my original 245/75R16 Dunlops and no canopy with odometer I was getting 19.3mpg.

If you have larger then stock tires don't calculate MPG based on your odometer. If I did base on odometer mine would be down around 0.5MPG to about 18.7MPG with my 265/75R16's.

*Edit*
My GPS factor was calculated at 0.973319. So for any odometer reading it was actually reading ~97.33% of the actual GPS recorded distance travelled.

 

This here.

 

Replace your cats first then do a full tune up including cleaning the MAF sensor and throttle body, new plugs, etc. Have them check the O2 sensors and replace if needed. They can also check the compression and do a leak down since you said it seems to be losing power.

Even after a fresh tune up including new plug wires and driving like an old man I only get around 15mpg in my '05 with ARB bumber/winch/sliders/camper shell/etc. With real gas i.e. non-ethanol, I can get up to 18mpg if I'm easy on the gas pedal.

 

Not a chance in hell.

 

I get about 18 mpg on an average tank. I have armor and carry lots of tools and have heavy load E 33" tires. Many factors come into play.

Replace your cats, put in new plugs and drive a couple tanks. Then get back to us.