Terrible Mileage--help me if you can

[RLDSL]

Synthetic gear oil in the diffs, front and rear and synthetic grease in the front hubs will make a difference. That heavy oil gives you a lot of rolling resistance. Making sure to kick your timing up from the old factory setting of 8.5 deg to around 9.7 deg btdc to compensate for the ULSD will bring your fuel mileage back up some and recover the power lost in the fuel change. Make sure your wheels are alligned properly or you are dragging rubber down the pavement. Grease the snot out of your Ujoints if they have been neglected, Friction eats fuel.

 

[Michael Fowler]

Thanks for all the replies, and the support that my mileage is not all that out of line.
Tranny is E4OD
Axle Code is C9
Hubs are manual, and "free"
Tire pressure is about 70 and bouncy
Bed is empty
Weight is 6800 lbs
Mileage drops a little ( estimate around 12 mpg) when towing the 7000 5th wheel.
How I drive--I may not be the slowest guy off the line at stop lights, but probaly the the next slowest. In town, I watch the traffic lights a couple of blocks ahead to match my speed with condition.

 

[janos1]

Wow!! I'm being cheated big then, I just bought my '90 and it's getting prolly 10 running empty and just back and fourth to work. The other night I hauled a trailer full of stuff and drove like a saint and maybe got 8. It has the E40D trans, but I don't know how to tell what the gear is in the back. All I know now is it's in OD at just over 35 MPH. It runs 2,500 rpm at 55 mph. I usually drive around 45mph so I don't hurt anything. I am going broke feeding her. Injectors seem to be doing well as no smoke. So any info would be helpful.

 

[LCAM-01XA]

Janos, 2500 at 55 means you're either not in OD at all, or your converter never locks or slips like crazy - I use my converter as a gear splitter, and with it locked while cruising in OD my engine turns at 1900 rpms while truck moves at 57 mph. I have 4.10 gears, and there ain't no way in hell you got any slower gears than I do, unless your truck has been regeared for running on very big tires.

Michael, looks like you got everything working like it should and you're driving like you should, at this point I'm outta ideas - sorry!

 

[Agnem]

Michael, what kind of RPM's are you turning at cruise? Best milage can only be obtained at 1800 RPM or under. 2K is really the breaking point. I see this easily on the PSD Excursion with the computerized MPG meter that it has. If I reset it right after a fill up, I get real-time readings which are dead on. Under 1800 RPM I can see upwards of 25 MPG (and that's with an 8000 pound truck!), but over 2K it will drop to 17 pretty fast. It averages 19.4 on my daily commute. What kills it, is the 4R100 and that stupid program that runs it. If that converter isn't locked up, not only can't it get out of it's own way, but the fuel milage can drop to 10 or worse. The E4OD is not to unlike that I would suspect. You could probably pick up 2 or 3 MPG with a standard trans. I've gotten 23 MPG with the Moose Truck on occasion (and before ULSD), mostly on I-80 (not the flatest road around either). The key to maximum MPG, is low RPM, no power loss in the drive train (read a solid lockup drive), low rolling resistance, and good fuel atomization (a clean burn), good timing, and free flowing intake and exhaust. Given your present readings, I'd say your E4OD is the weakest link in your setup. 3:08 gears could help, as long as they don't result in it running unlocked even more than it does.

 

[Shadetreemechanic]

Thanks for all the replies, and the support that my mileage is not all that out of line.
Tranny is E4OD
Axle Code is C9
Hubs are manual, and "free"
Tire pressure is about 70 and bouncy
Bed is empty
Weight is 6800 lbs
Mileage drops a little ( estimate around 12 mpg) when towing the 7000 5th wheel.
How I drive--I may not be the slowest guy off the line at stop lights, but probaly the the next slowest. In town, I watch the traffic lights a couple of blocks ahead to match my speed with condition.

C-9 is the 3.55 axle. I think you have room for improvement, as my current 94 is extended cab as well and as stated earlier I top out at 18, but general mixed driving seems to be 16.5-17.5 pretty consistently. The e4od tranny actually has a lower cruise rpm than the zf-5 and should deliver alittle better economy on the highway.
When I get some time to install them, I am going to get some injectors from typ4 and see if I can improve mine any more.

 

[RLDSL]

. I've gotten 23 MPG with the Moose Truck on occasion (and before ULSD), mostly on I-80 (not the flatest road around either).

That is an understatement. If you can keep even a remotely steady throttle foot on I-80 in PA , you deserve a medal That is no easy task trying to squeeze fuel mileage out on that road It's hard enough trying to keep from getting teeth marks in the steering wheel I don't know if they've bothered to fix it up any in the last 25 years but we used to run across there full speed trying to make delivery appointments, just praying that there wouldn't be too many broken springs on the other end that we couldn't make deliveries, steering with one hand while hanging onto our guts with the other .

For those who don't know, I-80 in PA has expansion joints wide enough to bend small car rims and shread caps with intense frost heaves combined with tight curves , tighter than interstate limits in places , along with short grades that are steeper than interstate limits in places. It's a non stop death ride rollercoaster run in a big truck . If' he's got 23 mpg out of the moose truck on that road, that is true throttle mastery

 

[Dsl_Dog_Treat]

That describes numerous Michigan roads.

 

[Agnem]

It's pretty good now, but you still have to put up with single lane constructions zones somewhere along the way. Seems they are eternally working on it.

 

[Goofyexponent]

It's pretty good now, but you still have to put up with single lane constructions zones somewhere along the way. Seems they are eternally working on it.

Yep. gotta take the hours while we can get them! lol Same thing around here...I thin the government engineers PLAN to have roads come apart so we can work on them. It seems we are always working in the same spots of the roads all the time!

But on a milage note. Our car, a 2004 Grand am SE with the 2.2 four banger...I know what you are thinking he said that he had a car, not a piece of crap Pontiac and I agree. This thing EATS gas like nothing I have ever seen before and it has only 70 000 kms on it.
The car gets 20 MPG driving around town....my truck gets 18.
The car weighs less than 3000 pounds, my truck is closer to 7000+.
The truck is a big 7.3 diesel, the car is a weed whacker 2.2 gas

It seems that everyone I talk to with the same cars is getting crappy milage, I am not trying to talk about grand am milage in general, but ask people with new gas and diesel rigs what their milage is....some cases the IDI's and older carbed gassers are doing better than the new EFI vehicles. Hell, my 1996 F150 long bed, regular cab 4x4 with a 300 inline 6 and a 5 speed got 24 MPG city/highway combined!

 

[janos1]

Janos, 2500 at 55 means you're either not in OD at all, or your converter never locks or slips like crazy - I use my converter as a gear splitter, and with it locked while cruising in OD my engine turns at 1900 rpms while truck moves at 57 mph. I have 4.10 gears, and there ain't no way in hell you got any slower gears than I do, unless your truck has been regeared for running on very big tires.

Well, I just got the numbers off the tag on the rear. So here goes. (V122C 3L55 10 2 9H08)

So... what do I have?? I would LOVE to get rid of the auto and get a 5-speed but I'm having trouble locating a donor.

 

[Shadetreemechanic]

That is a 3.55. If you are running 2500 rpms at 55 something is wrong. With that axle and tranny you shouldn't hit 2000 until about 70. My guess is your tach or speedo is screwy, otherwise you should have your tranny checked out.

 

[Diesel JD]

Unless someone regeared the diff or did an axle swap. My 86 came from the factory with 3.55s, at some point a 4.10 gearset was swapped in, and a few years ago I swapped that for SLE215's 3.55s. If your tach is screwy that is really bad for an E4OD, they need correct RPM feedback as well as a correct FIPL setting.

 

[fields_mj]

I have pretty much the same truck, and I think I have 4.88 gears, and I've been averaging 17 mpg too and from work. 24 miles one way. Half a 4 lane hwy running about 62mph, 30% a back country road with half a dozen sharp curves (50~55mph top speed), and 20% driving across the edge of town at 45mph. I keep it under 2200rpm most of the time, and I keep it in OD as much as possible. The lower gears have me about a gear higher than I would otherwise be. Unless I'm on an incline, I start of in 2nd gear, and make my turns in 3rd. I idle through town (30mph zones) in 4th, and by 40 mph, I'm in 5th gear. When I hit 55, it really feels like I should be shifting again, but I just don't have the $$ for a double OD.

I started out just under 16 mpg back in late July. New ball joints, wheel bearings and an alignment job brought me up to 17. I'm planning on changing over to Amsoil, but I'm doing the Auto RX treatment first. Just ran my first tank with the Auto RX and was very pleased to see that I got 17.6 mpg. We'll see how the next few go. I'll be going to all synthetics after that, and then going to a stock tire instead of these 12" wide gas guzzlers that I have now. I'm hoping that will get me up to 20mpg.

 

[david85]

I can understand going with 4.10 gears, but why would anyone even need lower gears if they are ever going to even touch asphalt. These engines have so much torque that I just don't see why you would need anything lower than 4.10s. I remember when mine still had them, 1st gear was pretty much useless most of the time.

As I've said many times before it you want to get good MPGs while running empty, you need to stay below 2000 RPM. Regear the truck or slow down because those are your only real options.