I've noticed that when I fill up and reset the mpg on the overhead, after about 15 miles, I am reading about 2 - 2.5 mpg more than I read when I'm below half a tank; Is this just some system quirk, or does keeping the tank full actually do something to the mileage? Doesn't make much sense to me......

 

you have to understand that it is giving you an average economy, you have take into account the idle time at the lights, in the driveway, the rich condition in the morning, etc. if I reset mine at the station 10 miles away and drive 40mph home it will say i get 40 mpg, then I start it the next morning and watch it drop. Most folks say the over head does not work but mine has never been more then .3 mpg off!!!!! I drive my truck sometimes hard sometimes not and average 21+

 

Is there any way that the air pocket on the tanks can throw off mileage? It seems that if you have heavy braking, or on a steep incline, some air could be pushed out of the pocket, and displaced by fuel. Does this make sense? -GB

 

you might see a guage fluctuation but mileage, the amount of fuel used in a given distance, is not affected by tank angle, hills will affect mileage but the air pocket is a constant variable overcome by the float on the sender. if you are getting air in your fuel lines mileage is not going to worry you half as much as pushing that 6000# tank to the fuel station

 

Going out on a linb here but it could be like the FASS. A full tank has less foaming from sloshing around so less entrained air. Less entrained air = better milage. More fuel in the tank = cooler fuel may also make a diffrence. Just a guess

 

Page 67 of my 2000 owners manual says:

"Average Fuel Economy (AVG ECO) Shows the average fuel economy since the last reset or the last 30 gallons of fuel used, whichever is most recent. The reading after a reset may take up to 30 miles to get a valid reading."

Even then they are sometimes not accurate. Your results may vary.

 

Check it manually. For the first 25K I found that the overhead unit was about 1.4mpg high.

 

Reset it while coasting downhill and watch it display 60+ miles per gallon.