[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