MPG to good to be true??

[MuddenJeff]

Recently recived the CTS for my 2004 F150 FX4 5.4L CAI and cat-back, with 3.73 gears and tires that close to being stock. I didnt update using fusion yet becouse of all the recent issues but I did flash my truck with lvl 2 87 oct, and the current tire size. At the time I didnt know about the squish factor but I will be messing with that shortly (multiplying value by .97). Before the programer I would resent my trip meter to zero on each fill up and calculate my MPG, I was getting anywhere from 15-17 tops. Now with the programmer, the programmer is telling me I am in the 24-26mpg range which sounds to good to be true. I have noticed the dash spedometer reads a few mph off from the programers speedometer (ruffly dash @ 63 when programer at 60 and it gets futher off the faster you go).

I know I havent yet adjusted for squish yet but am I missing somthing bigger? I never could figure out if the programer is set with the correct gearin setting (3.73). I am almost ready for the next fillup and I will beable to compare the manual calc to the programmer and see if it is tracking the same.

Will your programer re-programmer you dash instruments? i.e. your dash speedometer and odometer? or is that a something else? I noticed the trucks odometer doesnt track perfectly with the interstate mile markers- but if I can get a hold of a GPS unit I might beable to figure that out better.

Any feed back would be helpful, Thanks

[Longshot270]

Yes that is too good to be true and heres why,

1) If your tire size is currently larger than actual it will make the odometer read more distance traveled per tire rotation than actual. Make the distance traveled larger than actual and you increase the calculated mileage.

2) Never go by computer readouts for things like mileage. That needs to be only on paper because the Edge programmers rely on MAF sensor values and corresponding mpg values. It then makes a running average. Using the MAF sensor inherently brings in error. Usually making the mileage value larger than reality. When you lift your foot off the gas pedal and roll or start to slow down, that portion of time is counted as 40 MPG, WOT runs are usually around 5 mpg because you are moving while using gas. When you compare many 40's averaged with a few 5's and a general majority around 15, the 40's will pull the average up very quickly. For a silly experiment you can reset the mileage calculator and do nothing but WOT acceleration then roll (no brakes) until you stop. Do this for an entire tank. The average will say you are getting good mileage, your paper calculation will not agree.

The programmer influences the speedo/odometer by changing the tire size. Your odometer calculation is done by keeping track of tire rotations. The only way the truck knows how far you went is by the tire circumference you enter. That mileage you have calculated could be adjusted by going back to the trip distance, multiplying by the squish value (.97 or whatever is accurate) then dividing by the number of gallons used.

An easy way to get your odometer is to program the truck to a tire size you know is wrong, like 20" diameter. Drive about 10 miles. You can either use a GPS, map or highway markers. Then use this formula

GPS odo / Truck odo x tire size you entered (20") = Correct tire size.

Distance increases the accuracy of this formula. My programmer uses the actual circumference in MM so that also helps but it should still work on diameter since you are only scaling the value.