I know on most HD Class A engines, you wanna keep it at about 1800 rpms for best fuel economy. I turn around 22-2300 rpm at 65 mph, and on B100 I get about 12.5 mpg (injectors could use replacing, but can't right now) and get about 13.5 on D2.

NOW, since I gotta replace my tires, I was wondering if I could get bigger tires to kee the rpms down. I know, I know- bigger tires usually cause worse mileage, but if I can knock off 3-400 rpms and keep her running like you guys with only 2 rear wheels and 3.55's, wouldn't that get me closer to the 16-19 mpg you get? I say closer because I know my extra drag will always hurt me, but this 12-13 mpg is really pissin' me off!

 

With your mileage now, bigger tires won't get you anywhere close to 16-19 mpg.

Sweet spot for fuel economy is under 2000 RPM's. However, sacrificing aerodynamics and increasing rolling resistance to reach that RPM point doesn't do much in the mileage category. Sorry.

 

I wonder how much of a loss in hp&tq I'd lose if I swapped my 4.10's for 3.55's.....If I do the math, dividing 3.55 into 4.10, I get .86- multipy my hp 320 and tq 700 into, that gives me about 277hp/606tq. That is a helluva drop, so that can't be right. Anyone got an idea of a better set of numbers?

 

Danny, if your thinking of spending the money for a front/rear gear swap, why not consider the US Gear OD? that way you can keep the low gears for pullin and power and have the higher gears for mileage. If memory serves the OD gives you a 22% rpm reduction. I know mine drops about 5-700 rpm per gear in "high range".
GearVendor makes one also but having owned both brands, I highly recommend the US Gear.
look here.Dual Range Auxiliary Transmissions, Overdrive and Underdrive


Barney

 

I don't have 3.55's in my '99 - I think you could get them in the OBS trucks but I'm not sure about our PSDs. How many miles do you have on those Swamps 175/146's that they could use replacing already?

Bigger tires generally does mean heavier, but not always. More aggressive tread patterns seem to weigh more from my minimal research. I run 265's now, but when I had 285's I was getting better MPGs, but they weren't very aggressive. Have you taken your rear brakes off & checked to make sure your parking brake isn't dragging? Or any of your brakes for that matter? Check, clean, and regrease everything along your driveline, just to be sure.

What about your tranny fluid? How old is it? Synthetic or no?

 

That item is no longer in production. I had a rather lengthy talk with a guy at U.S. Gear not long ago. He wasn't happy about it at all. They don't even have many parts for them if you have one and need a repair.

 

Guessing you are running 235/75-16's?

 

I've got a set of 3.73's I just pulled off my truck this summer.
They arent new, but they are free if you are willing to pay to ship them.
Only thing is the front was a D50. Not sure what yours is.

 

With my 3.73s, 265-75-16s, and hybrid-singles, I can bust 22 MPG keeping it around 1900 RPMs. I'd be pissed if I was getting 12 or 13 on the highway.

You already have singles, so you should be getting better mileage than before. What size tires are you running now, and are you lifted?

 

I can't say for sure what you should do, but I will say this: When I travelled down the highway at 2300RPM's, my mileage was cut in around half compared to cruising at 1800ish rpm's. Obviously a chunk of that was due to wind resistance at the higher speed, but I would'nt be surprised if bigger tires gained you a bit.

If you really want mileage, get a set of adaptor wheels and put commercial rated tires on it. They are harder, last longer, and increase mileage.

 

My F450 at 65 mph (GPS confirmed) turns 2500 rpm and that is where I keep it all the time.
Even with 7000 lb payload I recorded this week close to 12 mpg.
I still wish I had an extra gear or dual range differential, since I am taking 5% grades with mid size load on the 6th gear, but seems different gear ratio is not extremely critical for fuel economy

 

Thanks everyone for the input, I'll try to answer the questions as best I can

- I think my injectors, or at least half of them are going out because it rocks pretty bad at about 1700 rpm in nuetral, my Predator says that half my cylinders are between .8 and 2.6%, guessing thats the contribution, but I still don't know what the hell that number means...

- it's running stock 235/85r16, albeit kinda bald

- height is stock

- can't afford an overdrive unit

- not sure what size my front diff is, but my rear is a Dana 80

Back to the injectors, like I said on B100 it's getting 12.5 pretty consistantly, and about 13.7 avg on D2. I think between some possible residual junk in the fuel system (over time) and getting a ton of water through them one time, that they may be damaged, leading to chitty fuel economy. I'd love to swap them for hybrids, or hell even keep the same size if I can get them rebuilt or cleaned. Already talked to Johnathan about that, and it would almost cost the same to buy a new set.

It's just frustrating dumping cash into fuel when I know I should be able to spend less. She's my daily driver, and I will never part with her, just want to get her healthier!

Doing the math, even at a lesser mileage, I save about $9 each fillup buying my buddies B100. 740 miles (D2) vs. 687 miles (B100). Not much, but will build over time. Now if I could increase my mileage even by 3-4 percent, that's even better, I just need to see what I need to fix.

How about the ICP, or IPR, could those bea factor? I got just over 228,000 miles on what I would believe are original sensors. I do have the 17* HPOP too. Also, I think I have a small up pipe leak at the manifold, may get to cranking that down eventually....

Please keep the ideas coming!

 

Are they leaking up by the colector aswell? I remember Dan (kwikordead) had an uppipe leak on his 550, and picked up 2-3 mpg? IIRC.

BTW, when I fixed my uppipes, with international ones, I didnt see any mileage change though. But Ive been keeping my rpms down.

 

The so-called sweat spot is the combination of RPM and BP where the BSFC is a minimum for the FWHP being generated ...and going to larger tires to reduce your RPM will increase your BP!

 

So what you're saying is that it takes more hp to run larger tires and therefore will use more fuel than any savings you might see from reducing rpm?