Why am i not getting good MPG?

[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.