My '98 is getting at best 12 miles a gallon. My mods are stock plate forward, as well as afc. Star wheel four rounds from all the way foward, silencer ring gone, waste gate plugged off, k&n air. Im not sure if the timing has ever been checked, or advanced. Should'nt it be getting better on the highway? Oh yea, it has 354 gears, and nv4500. Any help would be greatly appreciated.

 

I'm no wizard with 12V's (I owned one once for 2wks) but as on any diesel timing plays an inportant part in power and mileage, if you have never checked it you should, I've read they are known to slip!

My advice (dont take it the wrong way) ditch the K&N. on turbo powered stuff my opinion is they cant do the job, I'm going to get out my helmet and duck cause that comment is likely to fire up some guys

 

I'll stand up with you about the k&n not being a good filter that flows AND filters. Yes it will flow air, no it won't stop dirt. If people start beeking, just ask how well the k&n did on the ISO 5011 test.

http://duramax-diesel.com/spicer/index.htm

 

My stock 97 has 182K miles and I consistently get 29 mpg on the highway (empty at 70 mph).

 

That has to be a typo.

 

Thanks, i will address the timing issue as soon as i can. And for the k&n, it'll probably go to.

 

My mildly modded 98 12V gets 20-21 on the highway, at 70 or less.

 

Not a typo, I have nothing to gain by lying. Frankly I am surprised too.
I drive from MI to FL frequently, on I-75. I fuel up at exit 350 in Ringold GA and go all the way to St Petersburg (about 600 miles) without refueling, on much less than 35 gallons).

 

All depends on how you drive, dad is getting up around 25 with his 06 Megacab with a bully dog programmer set on performance. Course most of the highways he runs are 65 two lanes.
I can squeeze 20 out of my dually with 4.10 and the nv4500 if I keep the motor at or below 2 grand which is like 62-64 mph. I could see getting over 22-23 mpg if I had a second overdrive.
Check the timing, if the engine is running good and the timing is right than to fix your fuel mileage problem put an egg under the go peddle.
I also agree ditch the K&N they let to much dirt in, I am really happy with the BHAF set up flows plenty of air and still filters good.

 

Nick I agree 100%, I have been a mechanic many years and i can see the difference on motors that have run k&n for long periods......i tell em to just take it off and don't run any filter at all, it's about the same thing. Only argument i ever got back about it was from my kid......but 17 yo kids argue about everything. Suppose if ya took it off, cleaned and oiled it EVERY day it might work as advertised.Who is gonna do that tho?

 

What rear end gears do you have? My 96 has 4:10's (awesome for pushing snow, terrible for highway mileage). When I got it she did 16mpg, empty, or loaded, now several years and 100K later (rebuilt again tranny- otherwise see sig) she's down to about 12 or 13. Time to check the timing and tappets.

3:56 gears get 24mpg plus, 4:10's will never catch up with out help.

 

600 miles on 30 gallons is 20mpg. That is believable. You need to work on your math skills.

 

Back to the timing issue.
I got up to 25 mpg on straight freeway driving with 16.5° timing UNTIL ULSD. Now, the best I can squeeze is around 20 mpg. A buddy did some testing last summer and discovered his mileage dropped off after going more than15.5° timing.

 

That is pretty drastic. A 20% drop in mileage with a fuel that has only slightly fewer BTUs per gallon. I don't suppose you bought into the 2 cycle oil myth about the same time? I've noticed no loss in mileage with summer fuel ULSD (20+ or -) and maybe a mile per gallon loss with winter fuel. My timing has been set at 15.7 with both fuels and I use no additives whatsoever. Good way to fuel (pun intended) the internet rumor mill about ULSD.

 

I was trying to point more to keeping the timing in the 15° arena than to point a finger at ULSD.
I'm thinking it has more to do with a secondary issue with ULSD, cetane. ULSD is a nice rock bottom 40 cetane. Possible the 2% bio is not helping.
My towing and or hauling mileage is down 2-3 mpg as well.

I have no idea what the 2 stroke oil myth is about.