Thanks for all the in put!! sorry this has taken so long to post back, as I work out of town some what.
I put a 195 thermostate in the truck, down the tire pressure in the rear tires to 32psi(making them smaller not much though) My mpg jumped to 12.3 with nothing but a suit case and my shell on the back!!!! I think the thermostate was the big gas drinker. I pulled my trailer over the pass holliday got 10.1 mpg
Again thank you all for your reponces
PS my wife told me I fail to mention I drive 70-75 all the time

 

I agree with the tire size issue. Sometimes the tire size will need to be changed to optimize the gear ratio. It all matters on your gearing and current tire size. If you have 31's on your rig and are doing 1400 RPM's at 60 then switch to 33's you will be at about 1200 RPM's and this will actually cause worse mileage. However if you are at 2800 RPM's at 60 and throw on the 33's you will definatley gain in mileage.

Heck Just throw on a set of 44's, that'll give ya some mileage!!

(no pun intended!)

 

>I put a 195 thermostate in the truck, down the tire pressure
>in the rear tires to 32psi(making them smaller not much
>though)

Actually if you're comfortable with more tire pressure do it. It reduces the rolling resistance of the tire. Which basically means the rounder the tire the less power required to flex the sidewall as it rolls past the flat spot where the tire meets the road. This requires less fuel. Don't go psycho though, too much can affect tread wear. and those tires are expensive.

Also just for info's sake, I got a 92 f150 4.9 with m5od. Truck originally had 235/75/15 tires. Now has 245/75/16 tires. The new tires figure out to about 1900 rpm at 70mph with the pretty standard 3.08 gears. This is in .80 to 1 overdrive of course. Seeing as how I blew the whole rearend it's gonna get a set of 4.10 rear gears, which will give me 2500 rpm at 70 mph.

 

Take a look here.

https://www.ford-trucks.com/dcforum/suspension/1660.html

I've already explained why bigger tires will improve OVERALL mileage. Why would 3.08s give better mileage that 3.55s, they reduce RPMs. You'd be suprised how much fuel your engine will use at 2000RPM vs 1500, despite the load. My grandparents towncar has an up-to-date MPG gauge, it constantly tells you what your getting for gas mileage. Just look at the link.



Primary rig is Green Thunder:
95' F-150 XLT 4x4, 302, 5 spd, MSD 6A, Flowmaster American Thunder Exhaust, Sunroof, Clear corners w/ Diamond headlights, CD player with 2 10" subs and some 32" BFG Muds .

That's it for mods now, saving for a Stang. Check out my Gallery for a look-see.

 

https://www.ford-trucks.com/dcforum/suspension/1668.html

Look here too about air pressures.

There is an extent at while bigger tires put the engine out of the power band, and the weight becomes a factor, but if it's only a few sizes, than it's not a real problem. If you went from 29" tires to 44" tires, obviously weight is a factor and the gears need to be changed. The only difference between tire sizes and gears on the mileage, is the weight of the tires, and if it's only a couple sizes, we're talking 10-15-20lbs per tire, thats not a big deal.


Primary rig is Green Thunder:
95' F-150 XLT 4x4, 302, 5 spd, MSD 6A, Flowmaster American Thunder Exhaust, Sunroof, Clear corners w/ Diamond headlights, CD player with 2 10" subs and some 32" BFG Muds .

That's it for mods now, saving for a Stang. Check out my Gallery for a look-see.

 

Its not so much the weight of the tire that matters, its the rolling resistance it has. Taller tires almost always are wider and they create much more wind resisntance and contact resistance than narrow tires. I can load my truck with 1000 pounds and not notice a big degrade of power, but when bucking a 30mph headwind my truck shuts right down. 31" tires, however, is not enough of a difference to change much.

 

>>Stang
>>>>>>>>I repeat myself, you will NOT see a power decrease with larger tires

I assume you mean the opposite?


>>>>>>>>Even if you had 35" tires on the same 3.55 gears, you will see an improvement in mileage, but your OD would be useless and your truck will be slow. The reason people change gears for larger tires is to restore their power, not for gas mileage.

I am sorry to say, but that is completly false. If your OD is useless because of it no longer being in the power band and you take it out of OD what are you occomplishing? All you are doing is adding rolling resistance, air resistance, and weight as a result of the bigger tires. With OD off and 35's you probably would be about the same RPM as OD and stock tires. Also, engines run most efficient at certain RPM's. Trucks like my dads V10 dodge run best at around 2000rpm and only downshift on the most steepest grades and gets the best mileage. Other trucks like to rev more towards 2500rpm because they have their own power band in which they make the most efficient power per fuel consumption. its not always the lowest numeric.

I have 33's and there is a big indentation on the carpet behind the gas pedal that proves big tires suck power. Fortunately, I get about the same gas mileage as I used to in town since I am a fairly easy driver, but there is NO WAY at all that highway mileage is even close.

 

35s over the 29" stock tires, yes your right. But like, 29-31" tires, yeah the mileage will increase. The 35s was an extreme.


That post when u asked if i meant the opposite, yeah that was my typo mistake that i realised after, and posted it later making the correction.


Primary rig is Green Thunder:
95' F-150 XLT 4x4, 302, 5 spd, MSD 6A, Flowmaster American Thunder Exhaust, Sunroof, Clear corners w/ Diamond headlights, CD player with 2 10" subs and some 32" BFG Muds .

That's it for mods now, saving for a Stang. Check out my Gallery for a look-see.

 

I don't know why you think wider tires will make a difference in air resistance, the headwind you see at 30mph having your truck shut down is because, your truck is like a box going through the air, the tires are exremely minimal in the air resistance factor. I don't think rolling resistance adds up to as much as you think it does if it's only a few sizes bigger. I had 29" tires and overall I get better gas mileage with my 32s unless i'm towing something. Obviously say a 2:1 rear end ratio on a 29" tire will put it out of it's powerband, but 3.08s vs 3.55s will save mileage, and all it does is change the RPMs per gear. The only different with larger tires is weight, which isn't enough to overcome the increase you get for reduced RPM. And if you wnat to include rolling resistance, fine but it aint that much. You are entitled to you own opinions, but I have my own proof because I change from 29' to 32" tires twice a year. One of my problems is I have a hard time explaining things, so if things seem out of wack, like the statement from the post previous to this one, I probably just made a quick mental mistake and I appologize.


Primary rig is Green Thunder:
95' F-150 XLT 4x4, 302, 5 spd, MSD 6A, Flowmaster American Thunder Exhaust, Sunroof, Clear corners w/ Diamond headlights, CD player with 2 10" subs and some 32" BFG Muds .

That's it for mods now, saving for a Stang. Check out my Gallery for a look-see.

 

I know on my rig I went from the stock tires to 33's. I got worse gas mileage by about 1.5 MPG. There is a formula to figure this out. It's almost like the Steel driveline VS Aluminum driveline issue. Less weight with a driveline can increase HP and TQ by a large amount. It's called "rolling friction". I'm not an expert on this by any means but I know if you add weight (which my 33's on 10" rims weigh a ton more than the stockers on steel rims") you will reduce HP.

Now with 29" vs 30" vs 31" you can be right. However like most of the previous posters stated it all depends on gear ratio to power ratio. If you are running a 460 with a C-6 and 4.11's you will be right. But if you are running an 4 cyl Toyota with 3.08's and an automatic overdrive tranny you would be wrong.

 

 

I would agree almost under all circumstances that 31's would increase overall mileage. And that wind resistance probably isn't that much of a factor, (these trucks have such terrible wind resistance already) but I do think that tires that are fairly wide create more friction with the ground therefore slowing you up. Same thing with big lugs. Compare stockers vs. Swampers. Which would you want for MPG if size remains the same? If you stick with an all terrain 31" you will be set. MPG will probably go up a little bit but I wouldn't change sizes just for that.

 

Your right, it depends on the size and the gear ratio, you can't let it to far out of the power band. If you have 3.08s, forget it anything much larger will reduce power. However, as long as the final drive ratio is 3+, you'll increase milage if the tires aren't huge. I'm talking about 31-32" tires over stock 29" tires, sorry, I have a hard time explaining myself throughly. 33's weight ALOT more than 29s, so weight becomes a factor there. I notice a 1-3 mpg difference when i shift at 2000 rpm every gear vs 2500 rpm each gear. Those extra RPMs use more fuel then most people realize. Keep in mind, you're also going faster per RPM, thus increasing mileage. You might have went 35mph at 1500rpm in 4th where, now with slightly larger tires, you're going 35 mph at 1200 rpm. That in itself increases mileage, as well as the fact that the RPMs are lower. I think that those two factors, increase mileage more than rolling resistance and weight decrease mileage. I don't think you'd see even a 1 mpg difference between a 31" swamper and a 31" all terrain, sure there is more tire on the ground, but we are getting pretty picky arn't we? All i ask is that you be patient with me, it takes me a lot to get the information down on the screen in a way that you can understand.


Primary rig is Green Thunder:
95' F-150 XLT 4x4, 302, 5 spd, MSD 6A, Flowmaster American Thunder Exhaust, Sunroof, Clear corners w/ Diamond headlights, CD player with 2 10" subs and some 32" BFG Muds .

That's it for mods now, saving for a Stang. Check out my Gallery for a look-see.

 

>Unless you installed an aftermarket chip in the computor
>which calls for a cooler thermostat 160 would keep you in
>the overly rich mode. Try running a 180 degree instead. Your
>31 X 10.5's won't help either...they are a drag on a small
>engine like the 4.9
>Deen


Have you ever driven a 4.9L? The 4.9 will take larger tires like a champ with the low end torque. Took my 32" tires on 3.55s much better than my new 302 does.


This guy has 31" tires on 3.55 gears, thats perfectly fine, we're not talking 35s anymore and i'm realizing that we are taking away from this guys post, so maybe we should wrap this up or continue on another posting?

The 31s on 3.55s are not the cause of your mileage problem, look at other things. I appologize on my behalf for invading this post.



Primary rig is Green Thunder:
95' F-150 XLT 4x4, 302, 5 spd, MSD 6A, Flowmaster American Thunder Exhaust, Sunroof, Clear corners w/ Diamond headlights, CD player with 2 10" subs and some 32" BFG Muds .

That's it for mods now, saving for a Stang. Check out my Gallery for a look-see.

 

How could you advance the ignition, if the computorsystem is OBD-II? I have a 96 150 supercab 5.8 OBD-II, and as far as I know, it is not possible to advance the timing whithout a chip.Please let me know.