My old 3.55 semi floater finally gave up the ghost. The pinion was very loose when I got it about 4 years ago and I tightened it and have been running it like that. It finally got to the point where I could barely do 55 mph.

I looked around at the junkyards and all they had at the time were 4.10 FF rears. I figured that would be ok since I am running slightly larger tires(325-85/16) and carry heavy loads. I picked one up for $250 and brought it home. It was out of a 4wd and had the springs on it. I cut those off and put them in the metal pile.

I checked the backlash and was surprised to get .015, so I left it alone. The bearings felt good and were silent at all speeds with the rear end hooked up.

I decided to use 43-701 springs on the new axle. It rides a touch higher, but looks good. I can leave some slack in my leveling valve now.

I drove the truck about 40 miles and it was great! The truck averaged about 20 mpg. 1st gear is right where I want it, but I find myself looking for another gear once I get up to speed. I know the speed-o-meter is off quite a bit, but I can fix that.

The bad news is that I drove the truck about 40 miles yesterday and burned up a 1/4 tank!!! It was noticeably louder and I felt as if I was giving it more fuel. It was colder outside, but that shouldn't mess it up that bad.

What is going on? How did I burn up a 1/4 tank that fast?

 

You said it yourself. You feel you need another gear which means the engine is running at a higher rpm than it usually is which means it is going to burn more fuel.

 

what year truck and what engine/trans combo?
that's the problem with the lower gears.they pull great and make the truck feel much more powerful all the way around,but if you drive in 55mph or higher zones often,then 3.55's are the ones you want to stick with or she's gunna burn through the fuel much quicker.it's just the way it is.high rpms=higher fuel consumption.
if the combo is w/ an old direct drive trans,then you really wanna stick to 45 mph peaks with the 4.10's to see anything that would be considered reasonable (sucky for today,but "decent enough" back 20 years ago lol.)

whatever the f250 or f350 2wd or 4wd is,with whatever engine and trans....none of the combos will average 20 mpg w/4.10's.it was a calculation error.
the only setup of the 8th/9th gen era that could possibly average a 20mpg would be a basic f150 with 300-6 - mazda 5spd and 2.73 (maybe.maybe.3.08's) running empty on the hwy with no to few stops.65mph or less (for actual true average.not just a peak.)

 

What I want to know is why the first trip was 20 mpg and the next was barely over 10? The sound of the truck changed on the second trip. I was getting over 20 mpg with the 3.55 in there before it went out.

 

fill up error.the tank didn't get fully topped up though you thought it did.the next tank corrected the error and corrected your true average to be 15 mpg in this particular case.

you left us hanging about the truck still.why's that? punishment? lmao.
ok,so you must have a 7.3l diesel sitting in a srw f250/f350 2wd and you got 20 mpg peaks out on the hwy,long distances with few stops with 3.55 gearing at 65 mph? i dunno you left us guessing about all this,but you did.best i can come up with anyway.

so you swap in a set of 4.10's drive 75 mph on winter blend diesel with 4.10's and you haven't checked your tire pressure lately.you drop 5 mpg as a result......if my guess work is correct (and i must be close) then that would explain the drop.mystery solved lol.
maybe you had the stock 235's on it with the 3.55's and went to a wider tire when you swapped to the 4.10's.that would explain some drop too,but you worded it as if the oversize tires were on with each gear ratio,but i don't think anymore would be required to drop 5 mpg anywho.plus just 2 tanks isn't really enough to grab a true average either.run 10 back to back tanks and that'll give her pretty accurate.

edit.im a dummy.
.....double edit.....no im not (well ok,so that's still up for debate.) but i had the average right the first time lol.run a few more tanks though just the same.

 

Sorry, didn't mean to leave you hanging. It is a 250 with a 4.9, m5o2, 4.10 rear, 235-85/16's all around. The two different mileage results were on the same fill up. It was doing great until yesterday and the gas just disappeared down a black hole.

I have been trying to solve a rough running problem too. I checked the fuel pressure today and it is 55 psi but instantly drops to 0 when the truck shuts off. It drops to 45 psi when I put the vacuum line back on the regulator and runs rougher. I have replaced the regulator twice and it behaves the same way. I have also replaced the pickup screen and filter. I have been thinking I have clogged/sticking injectors.

 

You can't use the gas gauge to calculate fuel milage these things are not at all calibrated or linear, if that is what you're doing then there is your explanation. 20mpg from an F250 is also highly unlikely, that's typically only possible from a relatively lightweight 2wd reg cab F150.

 

I have calculated mileage from fill ups and I have averaged at least 25 mpg on a regular basis with the 3.55. I have figured out where each gallon is on the gauge and it is pretty accurate to within 3 mpg. I can tell right away when the mileage drops. I plugged a vacuum leak tonight, so maybe that was the culprit. I don't know. I will have to take another good look tomorrow.

 

I am throwing the BS flag. No way a truck of this vintage gets anywhere near 25 MPG.

 

MMM HMMM......you got to keep it tuned.

 

I find the 25 mpg VERY hard to believe as well. My buddy has a '96 F150 2WD regular cab, short bed 4.9 with 2.73 gears and a 5 speed. He grannies the truck and gets just at 20 MPG on the highway at 55 mph. His grandpa got it new and gave it to him for graduation. Has just over 30k original miles and runs like new...

 

You stated your speedo is off quite a bit in your first post so you're guessing at the speed you're driving, you're also guessing at the distance you have travelled because if the speedo is off the ODO is too, and you're guessing at how much fuel you have used using the fuel gauge. With all that guessing there is no way your calculations are accurate, and if you live in Canada and are using that silly imperial gallon you're numbers are not comparable to everybody else using the US gallon for calculations.

 

Bingo. There are so many inaccuracies here it's ridiculous.

 

Fix the odometer/speedo and come back with your fuel mileage result. If you were running larger tires than stock with the 3.55s, then your odo/speedo were both off then too for any mileage that you may have calculated and is not valid to use.

I'd expect your truck combo now with 4.10 gearing to get about 14 MPG combined driving. Maybe 17 highway. Maybe as high as 20 MPG or so on the highway with 3.55 gearing.

 

My odometer was not used for calculating mileage. I used known distances. I was getting 18mpg at 80-85 mph with the old gearing.