Okay Guy I am new to this forum, so if you dont mind I would love some input.

I have a 2001 F350 SRW 4x4 with a 8 in lift on 38" 15.5 Toyos. hypertech programmer. Now here is my question I started doing some research to see what would be best with the tire size I have and from I have come up with 4.10 to 4.56 gears would be the best for all around MPG and performance. Now im leaning more towards 4.10s, stock are 3.73. what do you think? Also if i change the rear to I need to change the front?

 

I would go at least 4.56's, and yes, you need to do both axles. 4.10's won't make enough of a difference for the amount of money you'll drop for the gear change.

Some good information and links in this recent thread:
https://www.ford-trucks.com/forums/1...eed-input.html

Welcome to FTE!

 

Using the calculators available on this site, going from 32s to 38s, you need to change to a 4.42 to restore the stock gearing, so a 4.56 would be as close as you could get. A 4.88 ratio might even be better.

 

4:88's...

anything lower then that is simply not worth doing.

 

Sooo what i am looking for is getting better MPG, evertwhere I have read says to stay under 2,000rpms on the freeway to get the best MPG, BUT if i take the gears up to 4.56/4.88's won't that take my rpms higher? I have the 4speed overdrive tranny. I think 4.56's would be the closest thing to stock settings (which I personally think would give me the best MPG, but I could be wrong). Correct me if i am wrong on anything, PLEASE.

 

...............

 

What was your AFSC? Just curious cause im a crew chief

anywas....

Sooo what i am looking for is getting better MPG, everywhere I have read says to stay under 2,000rpms on the freeway to get the best MPG, BUT if i take the gears up to 4.88's won't that take my rpms higher? I have the 4speed overdrive tranny. I think 4.56's would be the closest thing to stock settings (which I personally think would give me the best MPG, but I could be wrong). Correct me if i am wrong on anything, PLEASE.

 

Yes, 4.88's would raise your rpm's...

This....
(4.56's would be as close as you could get to a relatively stock gearing setup)

 

If you are concerned about fuel mileage, then get rid of the 38 inch tires and the 8 inch lift.

 

Even with a gear change, your mpg is going to drop. Turning over thse huge tires will cost a lot of fuel...You are adding a hellofalot of unsprung weight a d rolling resistance. With 4.56 gears your mpg will certainly drop, but wih 4.10 it would be worse and no power for driving. If you cannot afford 7-8 mpg, I would suggest no doing it

 

mpg may or may not drop but i bet your trans sure will appreciate it.

 

I can afford to have the tires and lift. And I bought the truck this way. I just want to try to improve the MPG...I mean there has got to be a way to do it. I was thinking Beans 6 position chip, regearing and going to single shot injectors. Explain how going to a lower gear would drop my mpg IF it will be close to stock?! Cause in correlation the stock 3.73s with 38in tires equates to about 3.55 or less. So if 4.56 is closer to the stock 3.73 when you take into account the 38in tires wouldnt my MPG go up? Just curious

 

you may see some benefit to lower gears due to the extra rotating weight of the tire combo. if your 38's weighed the same as your stock tires (they dont!) then a gear ratio close to stock would result in fuel mileage close to stock.

think how much harder the wheels are to get moving .... that tire combo weighs a TON compared to the stock combo and as such your mileage will always be lower with the big skins.

as others have said, 4.56 gets you back to the stock gearing.

you may benefit from adding a little gear, making those big tires a little easier to get rolling, thus keeping your foot out of the throttle a little on the take off and up hills.

if this were a gas motor i'd say put some 5.13's in it and be done. 5.13's would be roughly equal to 4.30's stock.


being a diesel, you may not want to go that deep. i think your best bet is somewhere between 4.56 and 5.13...... so, if it were my truck trying to achieve what you are trying to achieve .... i'd go with 4.88's and consider it a good compromise.

do what you think is right!

 

since you have the 7.3 diesel correct? you should get the 4.88 gears and nothing else. Anything lower and it wont do much of anything and anything higher and it will make the 7.3 be too close to red line. And like said above with 38" tires and a 8" lift no matter what you do isn't going to help with the mpg. the 4.88 gears are going to help the trans from blowing apart and keep the motor in the "sweet spot" so your gonna see a little increase in mpg but like you should already know its not gonna be anywhere near stock again.

 

forget about the overall size of the tire... whats gunna affect ur MPG is the surface area of the lifted truck pushing throu the air is higher now even more of a flying brick.

also add to that the sheer additional rotating weight of larger tires (esp over 35"s and taller)

ur asking for the impossible.. going to 38" tires and a 8" lift is NEVER going to get u better MPG then a stock height/tire size truck.. EVER