Huge decrease in MPG!

[H3Spartan117]

Hi,

I have noticed that in the past month or so, my truck is getting horrible fuel mileage then that it used to get. Here a few months ago, I was getting 14.3-14.5 around town. That is babying it big time. When I would treat it a little harder, I would get about 12.8-13.2 around town. Out on the highway, going around 70, it would get 17-18, depending on traffic. 80 mph, I would get around 16.

Now, in town. I get 11.9 and that is with me babying it. At $3.40 a gallon, this gets very expensive. On the highway, doing 70, I'll get little over 13 if I am lucky. I have no mods whatsoever on this truck. Everything is stock. The only difference is that I have 285/75/16 tires instead of the 265/75/16 tires. But I had a pair of Mitch. LTX tires that where in the 285 format when I was getting the MPGs that I used to get.

I made a trip up to AL to get some stuff for my girlfriend, and I was pulling a U-Haul back. It was the two axle size, 12x6. It was a little lower then the top of my cab. But coming back, I was getting right at 11.2 doing 70. Hell, I was getting 11.5 pulling 18K!

So something is major wrong! But what can cause this?

[dieseldan2005]

what engine do you have and where do you live?

[planeoldguy]

My mileage went to hell when my up pipe started leaking, took me awile to figure it out because I dont have guages, A friend that works at a dealer stopped by for a frosty cold malt beverage and we got to talking, popped the hood, He found it in less than 30 seconds.

[todd]

winter blend fuel does it every time #1 diesel has lower btu

[todd]

Originally Posted by planeoldguy

My mileage went to hell when my up pipe started leaking, took me awile to figure it out because I dont have guages, A friend that works at a dealer stopped by for a frosty cold malt beverage and we got to talking, popped the hood, He found it in less than 30 seconds.

how much did your mileage change after it was repaired ?

[dieseldan2005]

thats why i was wondering where he lived at. Cause i am dealing with the exact same problem right now. One month ago i was averaging 15.7 and now i'm at 13.4.

[planeoldguy]

Dropped from 19 to 12, of course winter blend is partially to blame, Got some new Fel Pro donuts that replaced the GM parts, Soon as it stops rainin I'll see how they work.

[dieseldan2005]

Originally Posted by planeoldguy

Dropped from 19 to 12, of course winter blend is partially to blame, Got some new Fel Pro donuts that replaced the GM parts, Soon as it stops rainin I'll see how they work.

What GM parts are you replacing? I guess i'm just confused

[todd]

was curious cause mine are leaking as well

[spardigus]

I just noticed a drop in my mpg. I normally average 16.9 a tank but on my last tank i got 13.7. I am gonna give it another tank or two before i make any assumptions though.