I drive a 96 f150 4x4 with the 300 straight 6 and manual transmission. I am currently getting HORRIBLE mpg(7-8mpg)
Truck is not stock, have done things that I know will hurt mpg and some that may help, I will try my best to name everything
6in lift
35 12.5 tires
Above cab height camper shell
truck is always loaded with tools
as far as I know these should be the only big negatives to my mpg
new parts list include (known functioning properly)
KN cold air intake&box
MAF sensor and duct to TB
TB and IACV gaskets
vacuum lines throughout
fuel pump, filter and lines
Fuel pressure regulator
Entire Spark system-motorcraft plugs, distributor(whole) upgraded cap/rotor, wires and coil along with adding gap to spark plugs for advancing timing( 6litre tune up)
many other parts that shouldn't effect mpg in either direction so I won't bother listing them.
Truck doesn't have any codes, I shift around 2500 rpm most of the time but never go above 3000, usually cruise between 17-2300 rpm, i got the truck 2 years ago and it was stock but only getting 12mpg, I feel like the added things could've costed me the 4mpg I'm losing now but I thought this engine was supposed to get around 16-20mpg. If anyone can help lead me to the next thing or something I may be missing it would be greatly appreciated.
God bless.

no cat straight pipe exhaust
all 3 02 sensors(back one spaced to avoid codes)

 

What gears are you running? With 35” tires you would probably want 4:56 gears. You’re using a lot more fuel to get 35’s rolling with the stock gears.

 

It has 3 55 gear ratio which i planned to update to a 4 10 this summer, but even when the truck was all stock(tires included) and empty i was averaging about 12mpg which is under par with what truck should be.
truck has 280k miles and with current gear ratio i hold 60mph at 1800rpm in 4th gear

 

Is the speedometer accurate? Was it recalibrated for the taller tires?

 

speedo is not accurate, actual speed is about 8-12mph higher than speedo depending on what speed its checked at.
would this affect how the computer decides fuel trims?

I have 2 separate GPS based speedometers in the truck for actual speeds, but if this would possibly affect function on computer i would have it recalibrated.

 

Are you using the odometer to calculate mileage?

 

The 4:10 gears would help, certainly give the motor more leverage and require less throttle. Where is the majority of your driving? If mostly 65mph or less, I would consider going with a 4:56. We just put 4:56’s in the wife’s Jeep as she wanted 35’s and it’s just over 2k rpm at 70 but really gets up and goes. I regeared an 85 f250 with a 460/C6 from 3:55 to 4:10 and instantly picked up 3mpg in city driving.

 

My driving is very mixed but seldom above 70mph,a steady mix of back roads and interstate driving. I dont think ive ever gotten above 85(shes slow)

after a little more reading i agree and will go with 4 56s. My lack of understanding gear ratios and power/mpg tradeoff led me to the 4 10s.

 

Yes. Plus the truck is travelling further than the ODO indicates which also makes fuel milage look worse than it really is.

At your typical cruising speed how much throttle does it require to hold it?

The biggest factor determining fuel requirements with EFI is engine load.. lower load = less fuel, higher load = more fuel. This is more significant than engine rpm.
A direct indication of the real time engine load to the driver is the throttle position, in general when the throttle is barely cracked open engine load is low, when you're really leaning into it engine load is high.
So absolute best fuel milage will be at the fasted speed achievable with the least throttle input, and note that may or may not occur in the top transmission ratio... much depends on how favorable the final drive ratio is to the overall weight and aerodynamics of the vehicle. The absolute worst fuel milage will happen when lugging the engine under higher loads at low rpm.

In your case the first thing you should do is recal the speedo, then do some tests to see what trans gear produces the best fuel milage. Depending on the results of that the next step would be axle gearing.

 

Adding to Paul's explanation of engine load impacting fuel economy, these trucks run in two different modes: closed loop and open loop.Open loop, oxygen sensor data is ignored and the injectors go into a high duty cycle, engine runs rich and fuel economy tanks. Closed loop, the PCM takes in the oxygen sensor data and modulates injector duty cycle to constantly switch engine operation from rich to lean, to average a reasonable fuel mixture for decent fuel economy. If engine load is high enough, you're in open loop. This is typically when you're accelerating, but engine load should drop once you're up to speed and go into closed loop. Trouble with 35's and numerically low gearing like 3.55:1 is that your always in a high load situation, and thus frequently in open loop operation. My 351W spends substantially more time in open loop mode now with 35" tires (4.10 gears) than it did with skinny 33" tires, and my fuel economy is notably lower. Easily 4 mpg less than when I had a stock setup.

 

+1 for the 4.56's
Your heavy cab over camper
I'd get that off of the truck until you actually go camping.
I had the identical truck a few years ago, no camper or tires and was getting a **** poor 13mpg
Generally speaking, you gotta flog those 4.9's to get them out of their own way

 

[QUOTE=Conanski;21461411]Yes. Plus the truck is travelling further than the ODO indicates which also makes fuel milage look worse than it really is.

At your typical cruising speed how much throttle does it require to hold it?

so i noticed this morning im holding 60mph at 1950rpm in 4th gear, to maintain that load throttle position sensor is reading about 47-52%(half throttle)

I am ordering the new ring and pinion gears tomorrow when I get paid, do i order the same ring and pinions for both front and rear dif? Im not finding much info on the subject, also would the front install kit(bearings,spacers, gaskets seals etc) be the same as rear? Also kind find parts list dedicated to front dif.
truck has the Ford 8.8 differential as far as I can tell.

 

Check out

LMR

Jeff's Bronco Graveyard

Good hunting.

 

Yours will have a Dana 44 in the front.

 

Gotta regear both axles or you won’t be able to engage 4wd without destroying something.