Just like the rest of you, fuel is killing me. My truck gets a solid 12-12.5MPG no matter how I drive it. The engine is a P7100 pump setup from an old Ford 2.5 ton(proving ground engine for the 2nd gen dodges). The factory timing is 12 degrees, and I think it must be part of my mileage problem since almost noone else I have talked to is getting this bad in the dodges. I have also read that 14.5-15 degrees helps with mileage.

So, what is the best you guys are getting for mileage and what is the timing you are running. What made the biggest gains?

Thanks!

 

If your mileage is that bad Id also investigate if your injectors are still in spec or are coked up and not spraying properly. Does the oil get black very quickly? If you make a lot of smoke thats a sign of incomplete combustion due to too much fuel or poor atomization of the injected fuel.
Maybe the p-pump is just tired and its not making much pressure?
Your transmission plays into this a lot too. Does that trans have a lockup clutch ?
other things that contribute are tire tread aggressiveness and tire pressure, dragging brakes, boost leaks and too much pre-boost fueling

 

4th gear in a 4L80E isnt very high, and I'm willing to bet your running 4.10's in that old GMC. That puts your rpms pretty close to 2k just cruising 60mph. If your going 70mph your looking at about 2500rpms. That alone will kill your mileage. As far as timing, yes advancing the timing 2-3* should give you a little more mpg. To much timing and you'll hear the engine making a metalic sound and you'll need to back it down a little. With alot of timing though the engine tends to run warmer with a big load, so if you tow heavy dont do it IMO.

Or if the mileage really kills you, trade me trucks. Mine gets 26-27 on the highway at just under 70 mph. I am a GM man myself, but love the old Cummins 12v's.

 

Thanks for the input guys, and I will answer a few questions.

The 4L80 is .75 OD, it is 4:10, and the truck has 35" mud terrains. So, it will cruise at 70mph at around 2K, but the tires and 6" lift dont help mileage. I do run them at 50psi. The tranny does have a manual lockup and I run it there most of the time. I have tryed 65MPH and it does not seem to matter. The truck has new injectors and the engine only has 150K miles, so I would think the pump should be okay. I was under the impression the IP was positive displacement so pressure should remain constant unless there was a cam failure....but I am FAR from a pump expert....real far. The engine does have an 18cm nongated HX35(industrial engine) so it does smoke a good deal at lower RPM. I have a 12cm housing that I am planning to swap. I suppose it is possible that it is running at too low boost at cruise and not getting a good burn.

 

18 cm housing is huge for a fuel plate and gsk. i would reccommend timing at 15-16 degrees. 12 cm housing should help out alot.

 

Maybe it's the letters GMC? seriously most fuel mileage problems are in the right foot, driving style is so important to mpg far more than anything else.

 

I agree to an extent....however, I used to just drive the heck out of it before this latest fuel price surge. Since, it has started hitting the pocket book hard so I started driving more conservative. I have also been making 10mile(each way) trips out to a house we are building almost daily. Previously, I just drove 5 miles to work and around town most of the time. The highway miles have made virtually no difference(maybe 15-20 miles per tank) in fuel consumption....which baffles me.

 

Yeah, if you frog it you can black cloud a lane of traffic due to boost lag...but, I dont do that with this liquid gold it now runs on. I need to back the AFC housing up, but I just have not done it. Instead I just drive easy....still get a little smoke on take off but nothin obnoxious.

 

Yikes, theres part of the problem. The gearing will put you over the sweet spot of 1800rpm by a bit. must be really low in the trans if those 35's didnt help lower the rpm at 65.
I spin 2200rpm at 70mph even with 33's. I dont want to run 35's unless I can get some decent 18" or larger rims so Im not running on balloons
My economy is only about 16mpg. Not good.
Using a mud terrain tire will take some economy too.. thats a more aggressive tread pattern and a lot of energy is converted into noise and heat.
If youve got new injectors then the tips shouldnt be coked up unless its been driven too easy for too long.
Seems to me the guys with the 215 pumps are the ones claiming good mileage. I cant get any info on any difference in injection pressures that might be the reason. If I ever get enough $$$ together I want to find out though.
I dunno how the full size GM's are, I only had a S10 before it disintegrated out from under me due to rust.
My truck handles like a drunk moose when I get into a panic stop or anything quick. If I was good at fabrication Id try boxing the frame all the way to the back and installing some crossmembers.

 

The truck handles and rides pretty good. I rebuilt the front end after I got it and that made a huge difference. It is much better ride than any 1T 4x4 dodge or fords I have riden in. However, in a panic stop it does walk back and forth a bit....I do try to avoid those.

I did have a TH475 non od behind this engine for a little while and it would cruise at 2700 or so at 70MPH....screaming I know. I thought the OD was really going to bump the mileage....I did not gain much of anything....which made no sense to me. The 4L80 is a hog on power, but the TH475 also takes some power to turn.

One interesting note is that the engine has mismatched injectors when I got it. There were 252bar and 300bar. When I matched the new ones I put 252bar injectors. I figured out after pulling them all that the factory installed injectors were 300bar. I tryed getting information here on the effects of changing and noone seemed to know definately what the effects were. The engine does run fine with the new ones, but that and the transmission swap and a head swap(cracked) were right about the same time.

I have heard talk of injection pressure effecting mileage, but I have a hard time believing that ~4300psi versus ~3600 psi will effect atomization or mileage....could be wrong.....

 

I think you need to bump the timing up to 16.5* and if that doesn't help a ton, you could raise the pop off pressure, although it is probably smarter to by some new era or DDP injectors.

 

Thanks. I am actually going to talk to a buddy of mine today about getting it in some time next week for the change.