Hey guy's,why do the overhead computers lie so much! The fuel mileage is way off. It was allways off 1.5-2mpg.but after installing my Hypertech the computer miles went up 2.5-3mpg but actually i gained 1.5mpg.Hauling our camper last week the overhead showed 15.8,but hand calculated it was13.2mpg.I just wandered why it's off so much. Thanks

 

It is because of the way that it is calculated now. It is not instant, it averages the trip since the last reset. I have had mine showing over 30 MPG when I reset it and coasted down a short hill.

 

There a few things to consider here:
1. Generally a completely stock truck will see very close numbers to accurate on the overhead if driven by the same person, the same way for multiple tanks.
2. Add injectors and the electronic pump delivers more fuel (to a point). This will throw the computer off a tad. 5% say.
3. Add a programmer like timing and duration box which fools the ECM and you get even worse overhead display because you are telling the computer one thing and doing something different. Expect another 5 to even 15% off.
4. Add an ECM downloader like the smarty and you might as well turn the display off as it is so far gone it doesn't mean anything anymore. Mine displays 33 MPG very consistantly but around town (stop and go, light foot etc.) the truck will get 18 if I'm lucky (hand calculated). Driving long distance it will hand calculate out to 23 to 25 but the display will say something TOTALLY different.

 

so we can say that the mileage left in the tank is off then?? if thats the case then I have gotten lucky the last 2 fill ups. i was running low but didnt want to spend $100 so i kinda let it go, which i normally do not do, but i was going by what the tank had left. Good thing i didnt run it dry!!

 

I'm not sure about that. I have been able to use that as a gauge very well and the lowest I've put it is 50KM remaining and filled 132 L of the 135 MAX.