Why am i not getting good MPG?

[F150 Man 4Eva]

Its me again. I searched for miles per gallon and find this thread. I just had a few question about my gas mileage. Before I start, let me say that I know that my 35" Nitto Trail Grapplers that weigh like 80lbs. a piece are a BIG factor in my mpg loss. Here are the facts..which within lies my question. With my mods (K&N FIPK and Flowmaster 50 series) and my 35" tires, but before I ever got the Gryphon...my MPG would vary between 12-14. Now when I got the Gryphon but only had canned tunes...I ran the canned 87 Tow tune. Mileage around town was about the same, maybe a little better. On the interstate however (going like 75) I was getting around 15. This made me happy. Now maybe I was thinking wrong, please tell me if I was, but I assumed when I got my custom tunes and ran the custom 87perf. that I would get better MPGs. Well going around 50, I can get like 16-17, going on the interstate (70-75) I am getting right at 11. Whoa now. What happened? Is the custom perf. supposed to give the best gas mileage? Let me also throw in that I am NOT out to get 17-18,hell even 16 Mpg, and I know I have a big 4x4 truck not a car. And I also know it is pretty much give and take with power and MPG. I just need to know if this is normal. Thanks yall...Travis

[F150 Man 4Eva]

Oh yea...before Gryphon...MPG probably not accurate seeing as how my odometer was thrown off by the tires. But with the Gryphone running canned tow and custom perf. I adjusted the tire size to correct everything. Just thought I would add that in there.

[Longshot270]

F150 Man 4Eva, what I think is happening to you on the highway is that the air resistance might be getting you. I'm going to use the metric tire sizes (235/70-17) as an example because the first number is the width between the sidewalls, I'll also use the sidewall ratio of 70 to find height. A 30 inch tire is usually around 235 mm wide but a 35 inch tire is about 345. Since the width has to increase with height then by going up 5 inches on total diameter then you add 11 cm or close to 4.5 inches in width. If you hold your hand out of the window going 70 you'll see how much wind resistance just a small area must overcome.

I'm also not a firm believer anymore that adding small amounts of height (like a 2.5 inch leveling kit) causes as much drag as I hear on other forums. I think the mpg drain comes from the bigger tires that come with the lift for most people. I added a leveling kit but my tires still had a few miles left so I didn't upgrade them. I still get about 17-18 mpg going 85 down the highway(average from gryphon before and after the lift)...and my truck isn't exactly what people call aerodynamic. Just the CB antenna alone causes enough drag to notice in a heavy head/crosswind.

The tow tune is designed to improve performance under heavy load. From the engine's point of view load is anything that keeps the truck slower than what the driver wants it to be. Watch the load next time your using cruise control on the road. When you go down a hill you'll see the load go down, when you go back up the hill the load will go up. Adding drag is the same thing as adding weight from the engine's point of view, it still has to work harder for the same results. When the engine works harder it uses more fuel. I'm not sure why the canned tow would do better than the custom performance though. Does the custom get better mileage in the city though?

[F150 Man 4Eva]

Hey I appreciate your help and explaination. My main question was about the MPG differences between canned tow and custom 87 perf. City driving seems to be about the same. All is good on the custom 87 perf. until I get about 60 or above then it really drops. This is watching Instant Economy. I noticed two big changes from the tow tune to the perf. My K&N is so much quieter. That drone is all but gone. The other thing is the TC locking up. Before when I would hit about 45mph in 4th, the TC would lock. Now I can get up to 60 and it won't lock. I pretty much have to let off of the gas for a second, let it lock, and then resume speed. I don't want to come across like I am complaining, because I am not. I love the way my truck and tranny are performing. Its just that when I was getting 15 MPG highway, I was excited. I wasnt getting that before the Gryphon. But now with the custom perf..I am way below that. So I don't know what to think

[Longshot270]

Well dont really consider the instant economy readings. The average economy is a rough estimate at best. Next time you get on the road switch the instant econ to average and see how those numbers look. And if you want to make the torque converter lock sooner you can go into the custom options menu and lower it 10 mph at the most.

[Jackpine]

4Eva, wind drag is really a very serious hit on your fuel economy. The difference in drag force (which is a constant load the engine must overcome just to maintain speed on level ground increases according the the "square" of the speed. So, a 17% increase in speed, which is what you see changing from 60 to 70 mph, works out to be a 36% increase in load on the engine.

Let's assume the drag force on the truck at 60 mph is 1000#. The engine has to generate enough power to overcome that 1000# just to keep you at speed. If you increase your speed to 70, the engine has to generate power to overcome 1360# (That's 1/3 more power needed - which equates to a significant increase in fuel consumption.)

And, if you increase your speed to 80 mph, the force you have to overcome increases by 78%! (More than 3/4 more power needed!)

Now, I'm NOT saying your mpg figures are reasonable. I'm just saying all this to "quantify" the effect of speed. I USED to get about 15.5 mpg if I cruised in the 60mph range, using the "canned" level 2 tune. Now, using my custom 87 economy tune AND with my DIY "Gotts" style intake, I've discovered I can get about the same economy in the 70 mph range. So, tunes and mods CAN help.

But - a headwind will absolutely KILL my gas mileage!

- Jack

[Longshot270]

Originally Posted by JackandJanet

4Eva, wind drag is really a very serious hit on your fuel economy. The difference in drag force (which is a constant load the engine must overcome just to maintain speed on level ground increases according the the "square" of the speed. So, a 17% increase in speed, which is what you see changing from 60 to 70 mph, works out to be a 36% increase in load on the engine.

Let's assume the drag force on the truck at 60 mph is 1000#. The engine has to generate enough power to overcome that 1000# just to keep you at speed. If you increase your speed to 70, the engine has to generate power to overcome 1360# (That's 1/3 more power needed - which equates to a significant increase in fuel consumption.)

And, if you increase your speed to 80 mph, the force you have to overcome increases by 78%! (More than 3/4 more power needed!)

Now, I'm NOT saying your mpg figures are reasonable. I'm just saying all this to "quantify" the effect of speed. I USED to get about 15.5 mpg if I cruised in the 60mph range, using the "canned" level 2 tune. Now, using my custom 87 economy tune AND with my DIY "Gotts" style intake, I've discovered I can get about the same economy in the 70 mph range. So, tunes and mods CAN help.

But - a headwind will absolutely KILL my gas mileage!

- Jack

Good figures. I knew the resistance was exponetial but I couldn't remember well enough to be sure. That 4x4 package must be really heavy for you to get those numbers because your truck is naked compared to mine. I've got the grill guard and CB that have plenty of wind resistance and I get 16-18 (by hand calculation) as long as I dont have to stop at traffic and lights.
But I also have custom tunes and the gotts mod. I'm thinking about making a ram air intake but with the 4.6L setup it is really difficult getting stuff to fit right. I wonder how that would affect my mileage if I ever could. I'll try it whenever I get a good design worked out.

[Power Hungry]

One other thing to pay close attention to is the timing. At lower RPMs with even a modest load, timing can drop to almost zero which can seriously affect fuel economy. This is done in order to help prevent detonation under those types of load conditions. Larger tires compound the problem by dropping the RPM at certain speeds.

For example, my Expy with 3.31 gears gets better fuel economy at 80 than it does at 75 because of the load and RPM range. Oddly enough though, it also gets good economy at 65 which, despite the lower RPM and load range, I assume is due to the lower wind resistance at that speed.

One other thing to remember is that in order to help reduce droning from the CAI kits, we remove the modified Cam Timing curves and revert back to the stock ones. This has a slight economy hit as well.

Take care.

[Jackpine]

Originally Posted by Power Hungry

One other thing to pay close attention to is the timing. At lower RPMs with even a modest load, timing can drop to almost zero which can seriously affect fuel economy. This is done in order to help prevent detonation under those types of load conditions. Larger tires compound the problem by dropping the RPM at certain speeds.

For example, my Expy with 3.31 gears gets better fuel economy at 80 than it does at 75 because of the load and RPM range. Oddly enough though, it also gets good economy at 65 which, despite the lower RPM and load range, I assume is due to the lower wind resistance at that speed.

One other thing to remember is that in order to help reduce droning from the CAI kits, we remove the modified Cam Timing curves and revert back to the stock ones. This has a slight economy hit as well.

Take care.

Yipes! This also means my interpretation of the effect of combining a custom tune with a commercial CAI is somewhat wrong! The custom tune actually DOES reduce the effectiveness of the CAI a bit due to the reduced timing. (I guess we could call it an unfortunate interaction). Reduced timing means reduced power, and, a corresponding reduction in fuel economy.

I'm sure glad I'm using my homemade CAI!

And, larger tires will mean lower rpms at any cruise speed, so, the reduction in timing can be an additional hit on top of the increased moment of inertia of the larger rotating mass as well as larger frontal drag due to more surface area and the increased rolling resistance of the bigger tires.

- Jack

[Power Hungry]

Jack,

Keep in mind that I am referring to camshaft timing, not spark timing. The spark curves don't change when running a CAI.