What Gives???

[JStraw]

Here's a strange mpg story. On average I am getting around 17 mpg at 70 mph. If I bump it up to 75 mph, I see it drop to around 15. 6 mpg. Now, here's the twist;



I was making a run from Nortern Atlanta down to Warner Robbins to see some friends. Traffic was jammed up in Atlanta and delayed me by an hour. Once on I-75 South I bumped the cruise control to around 83 mph to make up time knowing it would cost me in mpg's. But, to my wide eyed amazement, the mpgs did an initial drop to the 15 mark then started to climb back up. I hit the reset several different times and it always came back to the same reading. 16. 8 mpg at 83 mph!!! When I returned home to the tune of a 520 mile trip, I fueled up to the lip of the spout and read 30. 5 gallons on the pump. Doing the math, it comes out a little better, at just a tad over 17 mpg.



For the life of me I can't figure out why it would go and stay at 15 mpg at 75 mph but increase afterwards. Do you suppose the DC guys have programmed it that way to scare us into driving slower?



I'm on my second trip now with a return date of tomorrow, at which time I will see if the results are duplicated. And I've only got 4500 miles on the truck. (A/C was on also) Hmmmmmmmm?????



Edit: This is on my 04. 5 3500 laramie srw quad cab, not the '93 tank

Also, no reflash done.

 

[JStraw]

Well, I've made it back and here are the results:



Hit the interstate and cruised up to 70 mph. I then set the cruise control and reset the computer center. Mpg's settled into a steady 17. 3. After 10 minutes I increased speed to 80 mph. Mpg showed a drop to 15. 1 for about 3 minutes but then slowly made a climb until it reach 16. 4. Upon trip completion I refueled to the tune of 33. 7 gallons and covered 568 miles. That works out to 16. 8 actual.



If I cruise at 60 mph I'm seeing 20mpg. Question is... should I get the reflash done or not?

 

[LightmanE300]

Jstraw, did it ever occur to you that you might have had a tailwind in your favor? Just a thought. On flat roads with no wind if I reset my overhead doing 75 it says about 18. 5. If I do the same going into a headwind it can say as low as 15. 8.