Why Did The Mpg Decrease?

Harvey Halabookie · June 4, 2011

My father has an '07 NBS 2500HD Silverado LT Ext Cab with the 6.0L. He has just over 14K miles on it. He uses the truck mostly for working around his property and for plowing in the winter. About two years ago he put a Volant Power Core intake and a Hypertech Power Programmer (set to run on regular gas) on the truck. He also has stock size Goodyear Wrangler Duratracs. When he makes the 2.5 hour all-highway trip down to my house through the mountains, he always averages about 17.5 MPG.

He wanted a little more fuel economy with the climbing gas prices. I told him how I went from 17.5MPG average to 19MPG average when I go to his house by putting on my 3" Dynomax cat-back exhaust and K&N drop-in filter on my 1500. I also have Redline synthetic fluids all around. Last weekend we put the 4" Dynomax Cat-back on his truck and swapped the front and rear diff fluids over to Redline Synthetic. Now, on the same regular trip to my house, his mileage went down to 16.2 MPG. Any ideas why? He does not drive aggressively.

The new exhaust weighs about 1/2 of what the stock exhaust did. I saw a real increase in mileage on my setup, so he should have seen a similar increase. Do you think it has to do with the Hypertech? Or does the computer still need time to "learn" with the new exhaust? I think he is going to remove the Hypertech programming and see if the mileage increases.

From being on the forums for a long time, I guess the best bet would be to ditch the hypertech and get a custom tune (for him and me!). Any input or suggestions are greatly appreciated.

Thanks!

hockeypro1411 · June 4, 2011

My guess is that putting on a 4" exhaust system may have reduced the exhaust back pressure enough to hurt mileage. The hand-held programmer may also have something to do with the decrease in mileage, but I couldn't say for sure.

desrtrat · June 4, 2011

4" on a gas motor is to big. it's not the back pressure, you want as little as possible, but you decreased the exhaust velocity, the exhaust gases can't flow properly to help scavenge the cylinders..a 4" system is for diesel motors only...stick with 3"

good luck

sonjaab · June 5, 2011

I will have to agree that the new exaust system killed your mpgs.

These new trucks NEED back pressure to operate............

Another thought is do your local gas stations use the 10%

ethanol gas mix? If so that also is your problem.

The best my 07 gmt-900 2500 6.0 x-cab 4x4 gets on the hiway is

16 mpg and is bone stock so your not too far out of the ballpark

on mpgs.....................

If you wanna laugh my dads 06 Dakota x-cab 4x4 6 banger auto

gets a BIG 17 mpg on the hiway!

That thing is a PIG and can't get out of its own way and is super sloooow!

Harvey Halabookie · June 6, 2011

Thanks for all the input and advice. I guess he was doing really good getting over 17MPG on the highway before.

I never thought the 4" exhaust would be too much. That is the size Dynomax makes for the gas trucks. I wonder what size pipes other exhaust companies are using? Since we cut out the OE exhaust, I doubt he is going to want to put it back.

I guess we will see what happens after we remove the Hypertech, though setting it to "performance" mode may help with the bigger exhaust, but then he woudl have to use premium fuel. Maybe we will consider a Blackbear tune to tweak the settings and squeeze out some more MPG?

ATMINF · June 6, 2011

As stated before, with a 4" exhaust the decreased the exhaust velocity is not allowing the engine to "breath" properly thus the lower MPGs.

Big Whiskey · June 6, 2011

I've never heard of anyone getting close to 17.5 mpg with a 2500HD with the 6.0. That being said...stock gas engines need backpressure...with that 4" exhaust, the the exhaust gasses are basically just "falling" out of the engine.

ATMINF · June 6, 2011

I normally get 16-16+ MPG on the hwy with mine. I am guessing with the 6 speed and lower rear gear it helps. I would also say that if you can keep your speed at 65mph on the hwy you can get some good numbers. For me its hard to keep it below 70mph, I am usually at 72mph, that little bit does make a difference. Also I have a 4L80 and the 4.10 rear. I think the newer trucks have a 3.73 rear with the 6L90 trans.

Shaners · June 7, 2011

summer blend gas will also kill your mpg's

hockeypro1411 · June 7, 2011

summer blend gas will also kill your mpg's

I do believe it is winter blends that kill the mileage.

Harvey Halabookie · June 7, 2011

He was getting the 17.5 MPG on an all-highway trip, and he does not drive fast or aggressively. The 17.5 was according to the DIC, so not sure if it is 100% accurate, but it did drop over 1 MPG after the exhaust install.

I checked the other exhausts available for the truck (Corsa, MBRP, Gibson, Borla, etc) and they are either 2.5 or 3inch, so the 4" on the Dynomax does seem a little excessive. I am going to write to Dynomax and complain about it. I think I may help my father install a 4" to 3" reducer after the muffler to regain some of the lost backpressure and see if it helps.

In the meantime, he uninstalled the Hypertech and will let me know if there is any difference.