ok guys I have the truck complete or so I thought. The six inch lift is in, the new tires and bumpers are on. now the problem is I have lost a lot more than expected in MPG. it is a 2011 f250 crew with the 6.7, 4x4 lariat. I am trying to figure out what gears would be best to get this thing closer to factory. I know I am going to lose MPG but going from 18 to about ten was way more than expected, especially since these tires are only a inch and a half taller than stock. My 5.4 only lost about two mpg with the exact same set up, and the 35's on that truck were about three inches taller than stock. so I am noticing it has to run at higher RPM which was expected but for some reason the gas mileage is horrible. I am not interested in doing a delete, maybe a programmer but I am thinking the gearing would be the most effective. I am open to all input.

 

Few questions. You want the gearing close to Factory, what do you have in it from the factory?
How are you calculating your MPG? You have taller tires so a mile is no longer a mile to your truck, know what I mean?
If you tow anything heavy then go with some 4.30's.

 

Why would that be expected? It should run at a lower RPM for a given speed with larger tires. If you are running a higher RPM the transmission isn't shifting to higher gears or the torque converter isn't locking.

 

it has either 3.31 or 3.53 i havent looked at the specs for my truck in particular but thats what they came stock with. as far as RPM your right I meant that I was having to give it more gas to get up the cruising speed which was expected but its more than I anticipated if that makes sense. as far as calculating mileage I am just going off the trucks computer. I havent figured it out yet as far as actual but the gauge reading wouldnt be that far off due to an inch and a hlaf of tire. I do know one thing that my spedometer is reading right now. it was about four MPH off from factory. I think 4.30 is going to be a bit much, I was thinking 3.73. But again I am not sure what I have in there from the factory. tonight I will figure it out. appreciate the feedback.

 

one more question that i just posted on another post. does rim size matters or is it just tire diameter that matters. the reason i ask is because we accidentally ordered 17 inch rims. they fit on the truck just fine, which was the real concern. but if the rim size actually changes anything that might be feeding the issue as well.

 

With the wheels, you must think about the brakes when going smaller? It is a bit difficult without knowing what you have. Dont you have the window sticker from when the truck was new? Maybe in the glove box.
Might want to create a signature like what is below my threads. That will help with specifics when asking questions.
Re-read your above posts. There is nothing to really go off of. What was the stock tire size? What is the new tire size? What is the stock wheel size? You need to know the stock gear ratio.
Im not trying to hammer you, I am trying to help, but we got really nothing to go off of here.

 

Ok I just pulled the sticker, the original tire size was 275/70/18. The wheels fit over the breaks without issue. Like I said I messed up ordering the 17's. Stock gears are 3.55. I went to a 35x12.5x17. The new tires are only slightly wider and taller than stock. It is the lariat fx4 with the tenK towing package and camper package. 6 speed auto. And it had the rubber mat upgrade

 

Well I can tell you this. The stock size you had is what I am running with my 4.5" Icon lift. I just dropped down from my 325/65-18. I got some milage back when I tossed the bigger Toyo's to the curb. I have 3.73 and wish I had 4.30 when I am towing my toyhauler. I would think you will be taking a pretty big hit in MPG with the new tires. One reason is the weight of the tires. A lot more rotating mass.
My truck runs good like it is, tows good too. If you want more MPG forget the 325. I would go back to the 18" wheels, the 3.73 would put you close to what your factory setup was, except the rotating mass part. If you have a 4x4 you know this is going to be expensive. Hope this helps you some.

 

ok so last night i drove from my house to the intersection, which is exactly two and a half miles. I reset the trip indicator and it came out at 2.5 miles. so the larger tires did not throw off the mileage enough to make a difference as far as figuring my MPG. using that as an indicator it is off less than a tenth of a mile over two and a half miles. so with that being said the trucks computer for figuring MPG should still be on. so I am thinking the 3.73 should be good. I have a friend of mine regearing his f250 which has 3.73 in it. he said i could have his gears if they would fit in my truck. his truck is an 08. but i didnt know if that electronic locker in the rear would make a difference as far as those gears fitting my truck. any thoughts.

 

I have 315's on my truck and it reallt threw off my odometer. On a trip the odometer read 11395 kms but my gps read 12311 kms. Thats off by almost 10%.

 

Unless you are hand calculating you MPG, you really dont know what you are getting. I think you will notice a pretty big difference when you hand calculate. The trucks calculated MPG is usually off the mark. Try it on your next tank and report your findings. Download a GPS app on your phone and you can check a true mile.