Are you honestly trying to say your Yukon got more than 25 mpg? My gas light comes on with around 4 gallons left so if you're saying you get 650 mile on 22 gallons then you're apparently getting 29.5 mpg.

 

Yes he is. I remember that thread and even have the HPT tune he used saved on my comp.

Snake Eater; every 1500 HD I've seen has gotten shitty mileage. Don't know why either but all of them suck. What size tires are you running and have you done any logging or any tuning to it?

 

he has a yukon xl the gas tank is bigger than the regular yukon like around 30 gallons. but yeah he gets like 23 mpg when not towing. i say its the gears for one and the low *** overdrive the 4l80e has. his tires are 275/65's i believe. so around 32" tall.

 

I get around 24-24.5mpg cruising at 70mph on the highway. There has been extended trips with alot of fast moving trafic that helps with wind resistance and I've seen as high as 26mpg in those rare cases. Its a 31 gallon tank too, so I have pushed it to 700 miles a couple times but try to avoid running out of gas. lol

 

I had a 02 with 285's and it would only average about 13mpg. Those are big heavy trucks and that has the biggest factor for the shitty mileage.

 

not really because my 4x4 Yukon XL weighs just over 200lbs more than the HD. I think the 4L80E must be killing the power. 2700rpms at 70mph is too much. GM should have given the trans better gears so it didnt need such steep rear gears to get it moving, but still only have a half assed over drive.

 

My Dad's '99 2500HD(stock tires) would get 15mpg no matter what type of driving he did. My '05 SS gets 15mpg no matter what. I think the 6.0L trucks just like that number.

 

i would be happy with 15. Maybe I just need to tweek the tuning more. There just has to be more in it. This thing is the hardest combo to tune for some reason. Just dropping the PE to 2000rpms created lots of knock and ran lean. After gettign the fuel trims set and the WOT fuel set I tried to add just 2* timing and there it goes again, tons of knock and ping. Not to mention the stock injectors are running 96% duty cycle already, not much room for just simple tuning power.

Maybe the engine is the weak link? It has had regular maintenance but the first 16k miles was Houston driving and always light throttle. After I got ahold of it and opened it up it seemed to run smoother and a little better, but still not what I think it should. I always run 93oct in it too, never the cheap stuff, and RP motor oil.

 

You've got the 265 tires also, which might suck up a little more gas than the 245s that he had on his.

I did gain 1mpg when I put the dynatechs on my truck so maybe slap a set of headers on it.

Maybe shoot some top engine cleaner through it and see if that helps with the knock situation. If it has a lot of carbon build up from being babied around that could cause some knock, good to keep them blown out.

 

Yeah, I think it's the combo of weight, cubes and tranny gearing. Looks like it's time to whip out a turbo. Seriously, my mileage with a loose converter and much taller gears improved a little over 1.5 after the turbo, now over 17.5 average and 21 or so highway if I drive it right.