I have a '03 F-150 Auto 4x4 with 4.6 in it, 3.55 rear. Factory tires were 255-70R16. I put some 285-75R16 that are 33.1 inches on some 8 inch rims, and that really robbed some power, especially on hills. So I put a K&N filter and Flowmasters on, and that helped a lot, but the tranny still downshifts going up steep hills and overpasses. I'm only turning roughly about 1650 RPMs at 70 MPH now, which I think is too low. I'm also about to buy a Diablo chip and stick in there, and I know that will help with power, but I still think the gearing is wrong. So would I be better off putting in some 4.10 gears or sticking with the 3.55s, and if I go to 4.10s how much would it change my gas mileage?


Thanks,

Marlon

 

ppl over rate re-gearing. i swapped in 4.10's from my 3.55's and noticed no difference in power or gas mileage (the gears were free) IMO I would not waste the money to re-gear unless you are gonna go to 4.88 or 5.13. the chip wil most deiniftely take care of any power of gas mileage problems. stick with the 3.55's

-cutts-

 

I run 285-17x16s on my '99 with a 5.4 with no problems. I had a chip burned a few years ago for it, and I am running true dual 2.5" pipes (If you get rid of that junk factory "y pipe" after the cats it frees up alot of power). In fact I have a limited slip and can lay rubber thru 3rd gear with my pickup.

 

I agree with the gearing, I doubt you would feel the difference between 3:55s and 4:10s you would lose some mileage but I'd suggest 4:56s

 

Since you have overdrive, you could go 4.10, 4.30, or even 4.56 and still be able to cruise at a descent speed on the highway and get descent mileage as long as you aren't in the habit of flying at speeds over 80mph.

You say you feel a loss of power with the bigger tires. Re-gearing to 4.10 will bring back that power. If you didn't notice any loss in power going with the bigger tires, then you probably wouldn't notice much difference after re-gearing iether, unless you went lower than simply correcting for the tires. Everyone's experience is different. A guy with a diesel and 3.73's who doesnt tow a heavy trailer and goes from 31.5's to 33's probably wouldn't even know the difference. A guy with a 302 who goes from 29's to 33's and wants to tow his camper or boat would be well advised to swap gears.

I went from 3.55 to 4.56 with 33's in the F350 and it was a significant difference in power. Night and day. My truck is heavy and needs the torque multiplication at the axle to get it moving, so re-gearing was practically a necessity for me, plus I drive alot of hilly roads and tow with the truck on occasion, so a high axle ratio was just killing the transmission and the gas mileage with the bigger tires. I actually get better mileage now than I did before, because the engine doesn't have to work as hard.

I'm a firm believer in the right gearing for the setup and the situation. Simply slapping big tires and a lift on a truck is only doing the job halfway. Gears are what finishes it.

Motor modifications can result in increased power if done right, and I'm all for them, but usually don't result in increased mileage. More airflow = more fuel use.

 

Not to say you are wrong, because everyone is entitled to their own opinion, but I have a 2003 with the 5.4L in it, I was getting about 15mpg on the highway. After upgrading my exhaust and intake, I now get a little over 19mpg. After driving about 45,000 miles, my upgrades will have paid for themselves

 

Rambo, hey that's good news! Yeah I'm not saying it NEVER results in increased mileage, just not usually, at least in my experience so far. Maybe I just haven't stumbled upon the right combination stuff to make it happen....

 

I agree with SoCal,

Gears will put your engine back into its rpm "sweet spot" allowing it to work more effiecently and take advantage of the performance mods you have done. The 4.6 makes its power in a higher rpm range than the older V8's (302, 351 etc.) so I think lower gears will be a big benefit. I have a 91 f250 with 35" tires and a 460. When I changed from 3.55's to 4.10's I got better gas mileage and better response.