nagata

Alright, I've got an 08 XL w/ 4.2L & 5-speed. When it was bone stock (30" wheel/tire combo, 3.31 gears), I could easily get 19-20 MPG on the highway, 17-18 lowest with heavily mixed driving. When I upgraded to the FX4 rims w/ 32s, my highway went down to ~18 mpg, mixed was 17ish. I figured the mileage decreased because I had to give it a lot more gas to get the same acceleration, and had to downshift to 4th to climb hills and speed up on the highway.

So, to compensate for the tires and for an upgrade, I had a set of 4.10 gears and Detroit Truetrac installed. I figured the mileage would go down for highway, maybe 17, and probably be similar for mixed driving. But no, I was sorely mistaken...The first few tanks since the swap, I'm getting 14-15 MPG, max And that's 80% highway...I'm getting worse mileage than a V8, without any of the benefits

Any thoughts? I'm not beating the hell out of it either. That was my first thought, and I checked myself...Except for a instances of necessity (entrance ramps), I use light throttle on acceleration, and shift at or below 2500 RPM. At any speed, I get into the highest gear that doesn't lag, and cruise at 70 on the highway. While cruising at 70, it's sitting just under 2500 RPM, which, if I remember correctly, is only a few hundred RPM higher than what it ran at 70 mph when stock. A few hundred RPM higher equals a 4-5 MPG drop?

I'm not sure what I can do...Like I said, I expected a bit of a drop, but I don't know if I can live with this. 14, maybe 15 MPG on the highway with no load or trailer (and I have a tonneau cover!), I imagine it'll only get worse with a load I've thought about a tuner & intake, but I seriously doubt that would get back 2 or 3 MPG. It'll cost me another $400 to put the 3.31s back in (or $600 to go to 3.73s or 3.55s), and if I put the 3.31s back in, I've gotta go back to the stock wheel size, because it's just damn near undriveable when stock w/ the 32" wheels.

Ah, unexpected downsides...boo. Makes me a bit nauseous to think that I screwed myself so much, given the kind of mileage the new V8s get. I absolutely love my truck, but man, this doesn't feel good.

Larryw1

Did you calibrate the computer for the tires and gears?
are the readings correct?

I went to bigger tires on my 5.4 with 3.73 gears and I had to calibrate the speedo thru the computer programmer I use.

I went fron the stock tires to 35's and my miliage dropped from 20 mpg on the hwy to 13. I added the progrmmer and CAI my miliage went to 16 so overall I lost 4 mpg.
Before I put the performance stuff on my truck the MPG robbery was min 7 mpg. in stop and go driving I was getting as low as 10 mpg. on my truck it takes a lot of gas to get the 35's rolling. Stop and go sucks.
I knew that to do the bigger tires I would need to add power to my truck.

I don't know if your 4.2 can benifit from the exhaust ,tuner,cai setup. I hope some one that has tried the combo will let you know. I am sure some kind of an increase should happen.

nagata

Thanks for the input Larry! I had Ford try to adjust the computer, but they couldn't do it completely, so the speedo is off. Even so, I have a GPS, and I've used it to determine how much the speedo is off by, and it's approximately 10% high. Therefore, the odometer/trip meter is also high by ~10%, so I have have adjusted my MPG calculations accordingly. If I didn't adjust the miles, I'd think I was getting between 15 and 16 MPG. Perhaps an intake and tuner would do more good than I anticipate, but at this point, I'm not so optimistic.

Of course, the local dyno guy is trying to sell me on an intake, tuner, and dyno tuning (to the tune of another $850+tax, hahaha ), saying I should see a few MPG increase along with better performance. I'm sure there would be improvement, but it would take a long time to see the return on that money. Of course, if I keep this set up, I need a tuner to be able to completely correct the speedo...You'd think that, eventually, I would learn and stop pouring money into vehicles in the form of mods, but no, I'm stupid and fall for it every time I get something new, be it a car, truck, or motorcycle.

cody ryan

id just go back to close to stock and an intake w full exhaust keep the truck and if your really that worried about gas mileage get a car.

reginhild

Well, you know what will get you to 18 mpg from your current 15 mpg! Other options may or may not work.

Mod (Ret.)

Installing a tonneau cover does not help in increased MPG. There was a episode of "Mythbusters" awhile back where they installed tonneau covers, removed the tailgate, flipped the tailgate down, and installed a fishnet type tailgate, to see what, if any of these modifications would improve on MPG, and by how much. Their findings were that none of these improved gas mileage, with the exception of the fishnet tailgate; MPG went up by .03. They figured it was the weight removal of the tailgate that gave that. Pickup truck development has shown that the air vortex that is created when the truck drives "through" the air creates a back draft (circular) against the back of the cab/bed, pushing it somewhat forward. By installing a tanneau cover, this backdraft is broken, as well as dropping the gate, etc. Popular to belief, a tonneau cover will not increase your MPG. They found that leaving the truck bed as is will net the best MPG. If security is an issue, then by all means install a cover, but you'll not see improved MPG.

That new gear ratio is what's killing the MPG. That 4.10 to help spin the larger rim/tire combo is what's increasing the overall engine RPM.

Looks like you can't have the cake and eat it too. Keep the rim/tire and 4.10 rear end, loose mileage. Reinstall the 3.31 and stock size rim/tire combo, gain back MPG but loose the "cool" style your after.

Rainman

with a tuner, intake, and exhaust, you will get those MPG's back. when i put my intake on i went from 15 to 17-18. when i installed the programmer (edge), if i can stay out of it, i get up to 20 and ive even seen 21. i do have the 5.4 and 3.55 gears so you may not get the exact same results but it should be somewhere close.

f150cwcw

So if im reading this right, you only went up two inches in total tire/wheel size height? If that is true you went overboard on gears man. They usually but 4.10 gears in 3/4 ton gasoline powered trucks or for people that have 37" or taller tires on a 1/2 ton you might want 4.10's. But they are usually used on trucks with big lifts and big wide tires. Lets jut put it this way. I have a F150 4.6L V8 with 3.55 gears. I kept the same 17" wheels but bought bigger tires for my truck. I went to a tire that was 2.5 inches taller than my stock tires and i saw no noticable difference in acceleration or mpg. I understand that the 4.2 V6 is not quite as powerful as a V8 and that 3.31 gears are not strong but, 4.10's are a little extreme for that size tire. I would think that 3.73's or maybe even 3.55's would be plenty of power for your truck to turn the bigger tires/wheels and also bring up your mpg to a better number.

f150cwcw

The tuner, intake and exhaust will help a little but you still have 4.10's in the back. I would go the cheaper route first and try smaller gears than 4.10's but still bigger than 3.31's. An intake is like over $200 right? And some exausts are as much as $500, and not to mention the tuner, dont want to know what that cost. And you might see only a 2 mpg difference. The gears change would be way cheaper and he would probably see a bigger gain in mpg also.

nagata

Thanks for all the input, I appreciate all thoughts, views, and opinions, it helps me see the whole picture.


I have a 03 ZX2 5-speed that I average 32 MPG with, it's usually the daily, but it may have to become my g/f's daily sooner than later, which would put me driving the F-150 daily.


Fair enough. Regardless, it's a soft, so it's not hurting or helping.


Agreed, the 4.10s are exactly the problem, it's just more exaggerated than I had thought/hoped it would be. Learn, now I gotta either live with it or spend more money to change again.


While I agree that 4.10 is a bit steep, I decided on 4.10s after some discussion and calculations indicated that with the combination of tire size increase and 4.10 gears, I would effectively have a 3.83 gear ratio in comparison to my original 3.31. My original plan had been to do 3.73, but after doing the figures and coming up with this combination yielding basically a 3.55 ratio, I thought this wouldn't be enough of an increase. At this point, I would agree, I over did it on the gears. However, I would like to say that 4.10s aren't only put on lifted trucks with gigantic tires; engine/tranny combos and vehicles are different, granted, but Ford does offer 4.10s stock in some of the F-150s and even in the Ranger. Obviously, that doesn't mean the gears were the right match for my truck.

At this point, I'm not going to make a change immediately, I'll see if I can get any improvements. Since I probably won't, I need to decide what route I want to eventually go if I just can't live with the current status quo: go back to completely stock, sell off the FX4 rims/tires and (hopefully) the 4.10 gears, and spend the least money and be back to the 'best' mileage but also be back to crappy rims and tires, or spend more money, go down to a 3.73 ratio and hopefully find a good mix of power/driveability/MPG, go down to 3.55 and be back to roughly stock ratio with the downsides it had, or take a gamble on further mods that may or may not work...



Any value at all in used gear sets? I'm sure it's not much, but even if I could get half the cost of the gears back, that'd be better than nothing.