I have a problem with mileage going up and down by 3 mpg. I have no check engine light have been noticing this proplem for some time. I have a garmin gps and I brought a garmin HD which shows MPG also has a chart,when going down road the other day I had cruize on and one way it was running 16.5 MPG coming home was up to 18.8 to 19 chart showed straight line at 16.5 but on rough road you could see line go up and down the HD plugs into computer any help out there

 

If the Garmin HD works anything like the stock fuel economy gauge in the roof then it tells nothing but lies. The only way to get a good idea on fuel economy is to do it the old fashioned way, by hand. Fill the truck up drive a couple hundred miles and then fill it back up to Full. Take the miles put on and divide by how much fuel you put in. It's the only real accurate way to find out your fuel economy.

 

I have done the math and it does change the HD is more accurate than the overhead. The first time this happened was a year ago when I unhooked both batteries at the same time and went from 16mpg to 19.5 best I ever got then it went back to 16. I put a workhorse chip in and that did not make a difference in mileage more power. The HD has a graph in it for MPG and you can see it going from 19 to 16 every feel minutes just a problem that will one day show up.

 

Another thing to look at would be the road that you are on(rough vs. smooth). Did the wind change at all, gusts of wind could make your truck work harder ot keep the same speed.

 

The road did have rough spots in places, mostly good road wind was not blowing. I refueled at just under half tank had over 300 miles where before would have 230 to 240 I don't know what is going on but sure do like your help Thanks