Originally Posted by Brods
It is simple to understand why a aftermarket box will screw up the mpg accuracy, the ECM simply does not know that the box has held the injector open longer or raised the rail pressure allowing more fuel to go into the engine, so the ECM thinks less fuel was injected and displays higher mpg than actual. When you have a performance box any hope for accuracy is gone.
My truck will average 25 mpg at 40-55 mph on a fairly flat stretch with no stop and go.
Get over 60 and I immediately drop to 18ish. get over 70 and I drop to 16
thats a gas truck, my point is that diesels WILL get great mileage in the 40-60mph range. I don't doubt 17 mpg at those speeds. but you will not get that mileage towing a load at 70 mph.