Went from getting 19/20mpg mixed driving with the 4.8 to 15/16 mixed driving with the 5.3L swap. Didn't expect that bad of a drop.

 

You may wanna do a bit of tuning on that. My 5.3 gets usually 15/18 if Im nice to it. Have seen over 20 a few times.

 

Yea you need some tuning done. my dads 2004 ECSB 5.3 gets 18-21 mixed driving. i drove it to work for two weeks and very rarely got under 20mpg. and its got 215k miles on it

 

I'm getting about 19.5 on the interstate in my cammed 5.3

 

It is tuned.LOL It does have a built trans, mild stall, lift kit,4.56s and 33s though.LOL I did have a lift in 2008 with the stock trans, 4.8 also with 33s and I was getting around 17/18 mixed with 3.42s. Even with the loss, I wouldn't trade it. The power is great, chirping those big tires on the 1-2 shift and sometimes on the 2-3 shift. It runs pretty hard I feel. I will baby this next week and see what it gets.

 

Update your sig? haha

Chirping 33s is gonna be fun till the trans hates life.

 

Its built with all the best parts and has a great warranty, but swapping it and downtime would still suck. Its mostly at WOT so I just have to take it easy most of the time.

 

 

Are you sure you aren't just playing around more? Like you said drive lighter this week and see what happens. I would have guessed the millage would be similar. 4 mpg is a big drop and a bummer if it turns out to be the norm. I would think if feels like a new truck though.

My Tahoe get's 12 with a 3200 stall around town. Almost 20 on the highway. I would imagine it weighs similar to your truck but it is on slightly shorter tires then stock.

 

Apparently your 4.8 was a freak in terms of power output AND economy!