Why am i not getting good MPG?

[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.

[F150 Man 4Eva]

Wow, this is getting pretty technical. Is there any definitive answer to the question, is my K&N setup helping or hurting me? And after all of this, I am assuming that getting lower MPGs on my custom perf. based against the canned tow tune isnt alarming.?.? Thanks again

[Longshot270]

Originally Posted by F150 Man 4Eva

Wow, this is getting pretty technical. Is there any definitive answer to the question, is my K&N setup helping or hurting me? And after all of this, I am assuming that getting lower MPGs on my custom perf. based against the canned tow tune isnt alarming.?.? Thanks again

The K&N setup isn't exactly hurting you but to eliminate the CAI drone Bill has to go to a stock cam advance curve which isn't as good for performance. I would expect that the tow program would have modified cam curves for more performance while also having a richer fuel map to help on towing. With a CAI you usually have to use the richer tow program to keep from running a lean engine. The stock cam settings may be what are lowering mpg.

I also remember that you are running big tires. They are effecting the spark timing by effecting the RPMs.
As Bill said earlier when the RPM is low the engine load goes up resulting in the spark timing getting dropped to lower the chance of detonation. By lowering the timing you also lower both performance and efficiency. If the truck isn't regeared then your timing is getting thrown off because the RPM's aren't high enough. Also since you are slinging around more rubber the load goes even higher, dropping the spark timing even more.

[Jackpine]

Originally Posted by 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.

OK, so we're talking valve opening and closing times. Thanks for clarifying that, Bill. Forget EVERYTHING I posted in my last post, since I was thinking spark timing. I'm sure the valve timing has a somewhat similar effect, but nothing like what I was thinking.

- Jack

[Longshot270]

F150 Man 4Eva, if you want to try what tatsch said you can go in and adjust your 4th gear shift and lock points. Raising them will usually hurt your mileage and lowering them will do the same on performance until you get the RPMs back up, but every truck is different.