2007 Suburban 8 MPG HWY??!!

[FamilyGuy]

Good afternoon, everyone.
We just purchased this Suburban.
Specs;
LTZ model 1500
2WD
Completely stock
4.10 gear ratio (according to specs) 6.0L engine
124k miles.

No known or noticeable mechanical issues, except for some minor cosmetic items. We took it for our first trip and it Runs and starts fine.
Previous owner (original) said he had just changed oil, seems like new oil. He said all maintenance done.
Suburban is in great/excellent shape. We fell in love with it. It's got ALL the bells and whistles, but on a 400 miles hwy trip we're averaging 8MPG!
We know it's a tank, not expecting 20 mpgs, but 8?!
Is this really average with these specs?

What do you think? I'm hoping there's something wrong that I can fix.
Please give me some advice.
Thanks for reading!

 

[FamilyGuy]

Went to a local shop and he suggested I do a tune up.
I'll try a search here, but if anyone can tell me off hand what's everyone using for spark plugs on this 6.0?
Thanks.

 

[Big Buttons]

Spark plugs NGK TR5s or get the OEM AC Delcos but they are a little pricey

Make sure you clean the TB, MAF, and get a new air filter if dirty.

 

[FamilyGuy]

Thanks, Big Buttons.

 

[Dave01]

I would expect you to be getting 12-13 mpg on the highway. It depends on what speed, if you are driving 75-80 all the time I could see you getting down to 10-11.

Definitely try a tuneup.

 

[Goodinblack]

Check for any brake drag.

Check alignment and tire pressure.

Changed all fluids imo including differtentials

 

[FamilyGuy]

Tire pressure is good. I was doing over 70, closer to 80 most of the time actually. My older 01-03 Suburbans, were still giving me @ least 13mpg on the Hwy, @ that speed.
I'm doing a tune up this weekend and checking all the mentioned items as well.
Thanks.

 

[07Burb]

I would definitely suggest the OEM AC Delcos for spark plugs. May just go ahead and change the wires as well. 8 MPG on the hwy is definitely subpar! I get 16+ in my 07 and that's loaded down with all kinds of stuff. Also make sure you check your air filter. That can have a big impact on MPGs.

 

[FamilyGuy]

I got the AC-DELCO 41-110.
Should I get the OEM wires too?
I'll change air filter and clean out the components, Big Buttons suggested.
I'll be keeping my fingers crossed!

 

[07Burb]

Yes, go ahead and do OEM wires while you're at it. It's probably due for them anyway.