As of right now, with my 03 5.4 L FX4 sitting on 315-70-17 BFG's, i am averaging 10mpg. Granted i do drive with a bit of a lead foot. How would upgrading to 4.10's affect my fuel mileage? That ratio would get me back to about stock 3.73's, right? I have 3.55's LS right now. Please don't offer other suggestions for mileage, ie, "lower your truck back down and go back to stock tires" i want to strictly talk about gear ratio's here, thanks. will i see any improvement with 4.10's? Honestly, i would be happy if i could average 12mpg (what i got before lift and tires) with my lead foot and these gears.
If i am looking at the wrong gears for what i am trying to accomplish, please let me know as well...

 

consider going to 4.30 or 4.56 gears

 

x2
I'd lean toward 4.56.

Don't forget to reprogram your speedometer for those larger tires, if you haven't already done so.

 

4.30's would be ideal, however, from what i understand, they don't make them for both axles, this is a 4x4 i have. i would really rather go with the 4.56's for the power, but how would the mileage be? the reason i haven't recalibrated the speedometer is b/c it will be thrown off again when i change my gears, won't it? i hate to pay for things twice, and can drive with it wrong until i do gears. can the speedometer being off affect the odometer, or my mpg calculations? i want to eventually go with custom tuning, that is why i haven't already bought a chip.

 

Yes your speedo being off will affect your MPG calculations along with the odometer. Probably a 10-13% difference. Watch you speed too. At 60mph you are probably doing closer to 70mph.

 

i know from gps, that it is not that off. i gps'd my speedometer, at 65 displayed, i am doing exactly 70. i'm not worried about my speedometer right now. what i am needing to know is how deeper gears will affect my mpg, assuming my driving habits remain consistent.

 

I went 4.56s and wish I had done 4.11s. Excellent for towing but more than I needed. 4.11 would have done just fine. Matching to a tuner was key to learning this. Driveshaft speed is noticable as well. Wanted to be at 2200 cruising. Ended up 2400-2600. I get a solid 12mpg regardless now. hwy below 70/intown/towing. I am running 285/70/17s for street and 305/70/16s (33s) for mud.

4.30 are not available for the front.

 

Please comment further on driveshaft speed, hasbeen. Is it an annoying sound i can deal w/ or a potential safety issue that will require further correction? Also, what year, engine size, and are you 4x2, or 4x4? thanx!

 

I have a 98 5.4 4x4 with 4" lift. Started as a safety issue. (Ate up a yolk) the tailshaft/tailcone on my transfer case was worn and caused me a lot of trouble. I had to replace both the shaft and tailcone. My situation was unique and it had prior damage apparently. Now it woudl be just annoyance. The noise is not bad. The mudders just do cover it up. I have air bags on the rear. I let the rear sag as much as possible during trips to reduce. If I add air to the bags it will induce vibration into the truck along with the louder noise. Second new "balanced" drive shaft. So I know my driveline is good. But the lift and added angle do affect the setup. Higher driveshaft speeds will bring those features out.

I would look at driving plans cause you can't go wrong with 4.11s or 4.56s. I love mine for towing and around town. Larger than 33s = 4.56s. Shorter and for hwy I would prefer the 4.11s Expect better hwy mpg with 4.11 better in town with 4.56s. I am very consistant at 12mpg everywhere. But with the gear, hwy is no better. The 400 rpm change at cruise would have set it well. The 5.4 produces serious torque down low, tuner brought that out for me.

Many have 4.56s with no safety issues or complaints. Just sugesting if doing any hwy look at 4.11s. Easier to tolerate a gear to tall than to short. But in the end it is your preference that matters.

 

i don't do much highway, i have less than a 12 mile trip to work, and the farthest i travel is 100 miles to see my folks once every couple of months. therefore, i travel much more in the city, and i have concluded that due to this, as well as my tire size, i will be going with the 4.56's. thanks.

 

Get em in yet?

 

not yet, lol... money is not my friend right now...

 

I'm getting 4.10's in the rear diff tomorrow, but since I have the 5-speed, it's more like 4.56 in an auto tranny. This puts my 4.6 at 2,700 RPM's running 80MPH, which isn't too uncommon. Currently it's 2,340 RPM. Hopefully I get enough of an MPG gain at the lower speeds to make up for it some. I was reading a gear swap article for a newer 4v Mustang that was running the same RPM's (2,700) at 75...they commented that the higher-rev'ing 4v would love it, but if you had a 2v 4.6, it may be pushing it. Not sure why they said that...

 

2700 rpm isn't pushing any engine except maybe a diesel.

 

I'm running 4.10s with stock tires and rims. No issues. I'm up around 2400 rpms cruising at 68 mph.

I get great gas mileage anywhere... and performance is unmatched.