not sure how to post my tune, and I drive it really easy, ease off lights and try to keep a steady speed, I use cruise when I can as well

 

My 370 with 706 heads gets 20mpg at 68-70. It’s repeatable.

I run about 42 degrees timing in my cruising cells.

But, I have a 29.5in tire. Not a 34in.

Stop and go, no way. It’s 10-12. Daily drive back and forth to work I’m at 13.6.

 

I get about 12 mpg city in my truck, and 31 mpg highway













in my Integra. My truck just touches 14 mpg highway, 13.9-14.1.

 

My 14 with 3.42's averaged 22 @ 75-80 when new and 20 @ 80-85 on way back same trip when new. Now ~ 20 @ ~70 cruise. Mixed is 13.6 long term but I don't nail it all that much. Mostly putt around town loaded to 5800-6000 lbs. Still sucks. Traded my 06 2500 rclb because it averaged 13 loaded to about the same weight. Wish I hadn't because it would've been so much less to mod. My 07 turbo averaged 21 hwy on 32's and 4.10's.

 

Apples to oranges Mr DI and badass heads lol

 

I have a 2002 reg cab with a 4.8, 5spd, 244K miles, and I rev near max every chance I get. I get about 15 mix and gets 21 mpg road trip highway, 22 when I had a tonneau cover on it. That's going 75-80 with CC on.




Even a SRT10 gets 12mpg. OP got it doing that low mileage, and the previous owner probably got rid of it because of it. I swear I read this thread before, and this thread is only a day old..... I see it. https://www.performancetrucks.net/fo...r-what-555988/


OP, just throwin it out there. The 99 is a split year for the pickup, OBS and NBS were made that year. Any chance there is something that's made for the other model?

 

Not sure. I know it has the 99-2000 intake cover, unless your talking about pcm wise

 

If truck does a lot of idling, and stop and go driving that is not way off. One thing to check is make sure the knock sensors are not all crudded up and pulling timing, and egr valve is not sticky. I'd unhook or get rid of egr if you dnt have emissions testing. A front end of truck lifted with a Tbar crank can eat up fuel. Also are the tires really wide and heavy? I have a 06 Tahoe 4x4 with 32'' tires, 6,0L with 799 heads and small cam, 4.10 gears. In a small town with lots of red lights I get 11.5~12 mpg. Highway is 16~18 depending on speed, wind, etc. The stock 5.3L and 3.42 gear combo seemed to get a little better mpg in town but on HW the 6.0 and 4.10 combo is a little better. Weird how that works.

 

I can't really come up with something specific. If you got it that way, just thinking maybe the previous owner did some repairs and used the wrong parts for something. Do you have any previous owner history or receipts you might be able to double check part numbers?

Seems to me like most people in this thread are missing the part where OP said they were getting 10mpg when they got it and it was bone stock. The mods decreased about what the tune would increase.

Ummmm You're not driving this thing around all the time in 4wd are ya?

 

I've checked and hubs are unlocked while driving, idk what the previous owner could have done becausecive replaced mostly everything from plugs, fuel filter, air and oil filter, fresh fluid all the way around, no dragging brakes, new o2 sensors, only things I have not changed are injectors really, idk what else to check, faulty tps? Iac? Who knows, but 10mpg for when it was bone stock, and 9mpg after a full tune up with a tune just isnt normal, if you get what I'm sayin, I think it should just get more mileage as it is rather than me needing to dump several hundred dollars into upgrades