Will double check for codes and a burned out CEL, as well as check for vacuum leaks this weekend. Thanks. I have changed the fuel filter, plugs and wires. I still go back to the fact that with the stock tires I get almost 17mpg and it runs great. I to don't understand that this could only be due to the 32" tires.

 

What did the plugs look like. Were they all the same or were some cleaner or dirtier than others?

 

Physics...!

F = ma
45lb vs 55lb comes out about 25% with a ten second 0-60
T=fd
Increases this to about 1/3 (900 vs 1270ft/lb)

Then there is the power to sustain speed, rolling resistance changes(tread pattern, rubber compound) wind drag changes(more surface area, higher profile), and so on.. Yay physics!

I had a better post but my phone ate it.. You'll have to solve the full equation then apply the appropriate torque multiplier (final gear ratio..)

Bottom line might be cheaper and easier to use the throttle a little less aggressive and live with 12mpg instead of 8

 

I have 31", same gears, and running Flamethrowers Inj, I drive full throttle +50% of the time. I get 14-16mpg.
Physics point out that his engine is not running as efficiently as it should. The flow dynamics are off, before you start considering power losses due to bigger tires.

 

As you noticed, or maybe only unconsciously, I only address the issue of gearing. And that's just math. We don't know what tires he had or has i grabbed two random data points and worked some of the calculations. 15-20 is a pretty common mpg number for these with 4x4. Which he is getting.

We can correlate his mpg with the heavy larger tires with power requirements.. I'd check the pump/engine efficiency, IE plugged cat for a quick test...

 

Huh? Tires size is 32" as specified in the 1st post. What calculations are you running on the gearing? Newton's law to figure out fuel efficiency? Sorry, I'm a mech. engineer and consciously can't make sense of your reply

 

Ok and what do those 32" tires weigh, go ahead check your crystal ball I will wait, and what about the 225's?

Ok now how much torque does it take to spin them up to 60mph over a ten second time span, it's OK if you ignore the angular calcs and weight distribution, we just need a comparison point..

 

It can be mathematically proved that rolling weight on the wheels is equivalent to about 1.8x the same weight carried as cargo. So, if the bigger tires weigh 10 lbs more per tire, that's equivalent to 4x10x1.8= 72 lbs of cargo weight. So, unless you think that adding a 9 year-old kid as a passenger will cut your mileage from 17 down to 8, the tire weight is not the problem. Rolling resistance and tire pressure is a much more likely culprit - LT tires need more pressure to operate at same efficiency as p-rated tires. The stiffer sidewalls generate more heat given the same inflation pressure. You might try airing those LT's up to 40 psi for a while and see what happens.

That said, it's hard to believe tires and wheels alone would have the full effect that the OP is seeing. Something else is going on.

I run 33's on my '94 with 4.56 gears and a manual transmission. Mileage runs 18-19, with up to 20 on country highway cruising at 55 mph. Even with an automatic, 14-16 should be within reach with your current setup.