I have an 2004 super crew with 3.73 rear and I'm thinking about changing it to a 4.10 rear,anybody ever done this and if so, how do you change the gears Thanks: wlsmithjr1960

 

I'd jump right to 4.56 if you have anything larger than stock sized tires. It's expensive for 4wd since you have to change two sets of gears.

 

I'm running stock 17" wheels and the 3.73 rear is eating up my tire and milage, any suggestions? wlsmithjr1960 in nc

 

Then you surely don't want the 4:10's. Worse fuel milage.

 

by jumping up to 4.10's i gained around 2mpg.....

 

Thanks for the replys,but now i'm confused? ido pull a travel trailer with WD that weights 7800lbs, so whose right? should i or shouldn't i? Thanks wlsmithjr1960

 

If you tow regularly, you may need gears. If you have oversized tires and do more city driving, you may need gears. If you rarely tow, have stock 30's, and drive highway----you need to stop stomping on the skinny petal and/or get better tires.

 

i have 275/70/16 tires with 3.31 gears it sucks. i just dont want to put that kind of money in to a truck with 120,000 miles

 

I've read this before on this forum and I'm not saying I don't believe it but having a hard time wrapping my brain around it. Can someone please explain how higher rpms at any given speed are improving gas mileage? I'd love 4.10:1 gears but was afraid it would kill my mileage. I know you only burn as much gas as you allow by depressing the gas pedal but it still seems like higher rpms equal faster detonation equals more fuel per minute burned.

Please convince me 4.10 rearend is a good thing gas-wise!!

 

Short, short version:

Basically, bogging down an engine uses more fuel and is less efficient than an engine that is operating within it's power band. So taller gears bring your RPM's up to the more efficient power band area. But IMO this only applies if you're towing heavy loads and/or have oversized tires; both of which put additional strain on the drivetrain... So from a MPG perspective, if you're towing a 7500 lbs trailer with 3.55 gears (for example), then you're probably bogging down the motor, even with OD locked out. But if you were to regear to, say, 4.10 gears, then your motor would be operating at a more efficient (higher) RPM while at the same speed. Thus your MPG would actually improve with taller gears.

 

If it was, then it would be the standard gear ratio! ha, ha....

IMO, 4.10's theoretically will get you better mpgs while towing/hauling a heavy load compared to taller gears.. Not necessarily running empty however... City driving is different then hwy driving (obviously), so you might get better mpgs empty with 4.10's in the city.

Think of when you ride an old 5 speed bike. Put that gear on like '2' and you are peddling pretty fast on the flats, but hills and taking off from a stop is pretty easy, huh... Put that gear in '4' and do the same thing and you can go pretty good on the flats, but hills and taking off takes more effort...

Now, if you put that gear in '3', it's overall "not bad".... Add some weight to your bike or hitch up that 'Radio Flyer' wagon with some weight in it and you will be wanting to change gears!!

It's all a compromise and what are you willing to 'give' in order to 'take'....

I would love to have some 4.10's when I tow, but I like my 3.55's for the 90+ % of the time I'm not towing or hauling anything..... Compromise is what I'm doing.... Deal with it!! ha, ha!!

Mitch

 

thanks for the reply,Tire size is 255/65/17 and drive short distances most of the time and i'm not a pedal pusher. i only getting around 12 mpg when not towing and around 9mpg when towing my camper. any suggestions? thanks wlsmithjr1960

 

So, are you looking for better towing mpgs or empty mpgs?

I get an average of 15 mpg empty with 80% of the miles being hwy. I get an average of 10 mpg towing.

It's gonna be a compromise either way you go.... Right now, I've got 265/75/16 and 3.55 gears... If I had 3.73 gears, it would be the same as if I had the stock 255/70/16 and 3.55 gears.

Not really enough for me to worry about, so that's why I'm sticking with my 3.55's... If I had 3.73, I'd be just as happy I'd think, and I would probably not see any diff in mpg's either way.

It was what it is and you gotta do what you feel you need to do, but I just don't see you getting that much of a change either way..

Mitch

 

All those numbers sound normal to me. First, engine/tranny/cats do not warm up for several minutes so the engine does not operate in closed loop control. So those short distance trips hurt. Also, towing 7800 pounds @ 60+mph is a lot of work. 9mpg is average travel trailer towing fuel economy for any F-150. You'd have to step into a diesel to do better with that much wind resistence and weight.

RobertmII,

Galaxy said it best here:

Engine RPM is a very very small piece of the puzzle. Engine load is much bigger. Before the gear change yes, my rpm's were several hundred lower but the gas pedal was half way to the floor to hold a speed. (that's an exaggeration...but you get the point) If you so much as moved the gas pedal to pass or accelerate, the converter would either unlock or she'd downshift. Now, I can accelerate pretty much normally from as low as 40 mph IN O/D with no problems. I use probably 1/2 the gas pedal I used to use to accomplish a given task. That's your gas mileag and that's where you're getting wrapped up in the wrong idea. My 2-3 mpg increas is half and half city/freeway driving. I'd like to run out a full tank on the freeway...I bet I'd see a 5 mpg increase based on what I've seen so far!


The engine controller's fuel useage map (actually, tables) mainly are influenced by engine load, or requested torque. As the driver uses more accelerator pedal, more air passes the mass air flow sensor. Once engine/trans/cats are warmed up, the engine goes into clsoed loop control and almost always keeps the air/fuel ratio @ stoich (14.1:1). So, the more air, the more fuel used.

If the engine does not make as much torque at 2200rpm as 2500rpm, then a driver could use less accelerator position to get similar acceleration with shorter gearing. So, certain driving styles/conditions do actualyl get better fuel economy with shorter gearing than taller. That's engine calibration 101.

Specifically for F-150's engines and effective gearing (tire sized/gear ratios), they are happiest around 3.73 gear and 31" tires. So, go to 35" tires, you have to compensate by going to 4.56.

 

again thanks for the reply, i don't know much about tire sizes but if i'm running stock 255/65/17 what other size choices do i have and not have to do a lot of other stuff because of bigger tires? thanks: wlsmithjr1960