Sudden drop in MPG?

http://imgur.com/a/zve0s

2001 3.4 manual
250k miles

SO a few months back I made a huge move from my hometown. 700 miles trip with the bed fully loaded with most of my belongings. I made the trip easy and got about 16mpg. Im okay with that considering the huge tires I decided to run on the vehicle. HOWEVER, I recently went back for Christmas and got 12mpg on the same journey, with no load, empty bed. Any ideas on why that would happen so drastically? I replaced the air filter before I headed back, that didn't seem to have any effect. Next is spark plugs.

Thanks for any help!

 

Winter blend of gasoline

 

Winter gas usually only costs me about 1 mpg.

Did you drive the same speed? How are you calculating gas mileage? On the recent trip did you get the same mileage in both directions?

 

You didn't run the same exact trip, you ran it backwards. Could it be that the 700 mile road trip the first time had more of a downgradient, so that upon return, you're fighting gravity?

 

Or a strong headwind?

Other things to look for....air filter? Did you have the shell on for both trips?

 

headwind and slope were my two guesses. wind...especially on the plains, since your profile says nebraska.

 

the trip was from Colorado to Nebraska, mostly flat plains.

yes, got the same mileage to and from Nebraska on the 12mpg trip. wasn't a particularly windy day either travel days.

 

Sounds like a clogged catalytic converter to me.

This wouldn't throw any codes, but you have symptoms of a clogged cat.

 

Even your loss of power wouldn't be all that noticeable, but the gas mileage is.

Plus you had the engine and exhaust system at full operating capacity for an extended period of time. This probably caused the system to clog.

 

Interesting, I wouldn't have thought of that. I had a muffler replaced early last year, but that's all. Nothing ahead of that, i'll look more into that.

 

Even a new muffler will allow excellent flow. So much flow the cat probably couldn't keep up.

I rode around scratching my head at a loss of gas mileage and I did some "power tests" and the truck was just "not up to snuff".

Turns out I had a catalytic converter service call on my truck. Dealership fixed it for free.

Since then I've noticed gentlemen like you with the EXACT same problems. It's a simple fix, with tremendous results.

 

Pickups get gallons per mile don't they? 12mpg is fairly low but not unheard of.

 

That all depends. Guys with huge tires, lifted and have done the "right foot mod" etc.. probably don't get great mileage, but there are plenty of people here with 4x4's getting 18+. I easily got 28-30 in my older 5 lug. My old 4Runner with 32" M/T's and stock gears got 12mpg.

The cat is an interesting theory. I'll have to look into that for my truck. I was hoping that my new timing belt would increase the mileage, but that's looking to be the case... Years ago I got 18mpg towing a small camper trailer from Cali to northern Washington and back, but recently I've been noticing more like 14.5mpg on my commutes.

 

i got 20.6 in may on a pure highway commute. Most recently, best I could do driving very conscious was 18.6. I went from stock 31x10.5R15s to 265/75/16s, and winter fuel blend.

 

With an SC? That's showing quite a bit of right-foot restraint!

 

yea I think a lot of guys on here find the S/C has marginal impact on MPGs unless you are cranking it- i generally only use it in certain limited situations (e.g., getting the F outta the way of something, merging in front of an idiot who doesn't provide sufficient space, etc..)

 

yeah I def don't have a supercharger but im def getting the mileage of one. I got spark plugs waiting for me at the dealership. Back in the day I had a jetta with a cat failure, it was quite evident when I placed my hand at the pipe tip and got slammed with debris. That's going to be the first thing I try when I get home.

 

Quite the opposite, I went from 1000ft to 6000ft with the fully loaded truck and got the 16mpg. Went the other way and got worse mileage. And wind usually goes west to east in the region so it should have helped me.

 

hmmm, maybe the cat then as said above. Let us know what you find out!

 

When I went to larger tires and a topper the extra weight I'm sure made me lose a few gasahol points. But who cares right? Looks cool as hell! Lol. I have a full tune minus a new cat and 02 sensors.