I bought a used 03 2500 and its got some big tires on it , these things are huge. Will this throw off what the trucks factory MPG counter reads?

Yes... Oversize tire will effect the overhead computer. The bad part is there is no way for you to recalibrate the output (Flow rate GPH) of the fuel system after changes. So you might be able to correct the speed and distance but you have trouble with flow rates.

The OH is a guess based on injector pulse width, but even on a 100% stock truck it's not very accurate.

So to put it simply... Miles pre gallon figures are produced from 2 numbers flow and speed.

MPG = SPEED MILES PER HOUR / FLOW GALLON PER HOUR

22 MPG = 55 MPH / 2.5 GPH

So if the ECM is feeding the overhead computer bad information on either speed or flow then the whole thing goes south.

24.4 MPG = 55 MPH / 2.25 GPH (-0.25 offset error)

25.3 MPG = 57 MPH (+2 offset error) / 2.25 GPH (-0.25 offset error)

So in a few examples here I was able to skew the MPG display by a full 3.3 MPG with 2 small errors.

So would that mean I could use MPH(SPEED)/ Flow rate that my lift pump is rated for? = MPG ?

So to put it simply... Miles pre gallon figures are produced from 2 numbers flow and speed.

MPG = SPEED MILES PER HOUR / FLOW GALLON PER HOUR

22 MPG = 55 MPH / 2.5 GPH

So if the ECM is feeding the overhead computer bad information on either speed or flow then the whole thing goes south.

24.4 MPG = 55 MPH / 2.25 GPH (-0.25 offset error)

25.3 MPG = 57 MPH (+2 offset error) / 2.25 GPH (-0.25 offset error)

So in a few examples here I was able to skew the MPG display by a full 3.3 MPG with 2 small errors.

--- Update to the previous post...

Or i guess actually I would need to to determine the flow rate of the lift pump by the PSI its producing ?

So to put it simply... Miles pre gallon figures are produced from 2 numbers flow and speed.

MPG = SPEED MILES PER HOUR / FLOW GALLON PER HOUR

22 MPG = 55 MPH / 2.5 GPH

So if the ECM is feeding the overhead computer bad information on either speed or flow then the whole thing goes south.

24.4 MPG = 55 MPH / 2.25 GPH (-0.25 offset error)

25.3 MPG = 57 MPH (+2 offset error) / 2.25 GPH (-0.25 offset error)

So in a few examples here I was able to skew the MPG display by a full 3.3 MPG with 2 small errors.

So would that mean I could use MPH(SPEED)/ Flow rate that my lift pump is rated for? = MPG ?

No. Its the flow rate out of the injectors in to cylinders.

so if I was to do that then...

0.3 MPG = 55 MPH / 150 GPH Lift Pump

As you can see the math doesn't work out at all...

Or i guess actually I would need to to determine the flow rate of the lift pump by the PSI its producing ?

Here is a live display of my ScanGauge and its calibrated for my truck...

http-~~-//www.youtube.com/watch?v=SRQdoPs_tLo

Now you'll see the math in real time... My offset in the SGII are 22.5% GPH offset and 1% offset for speed.

Oh ic that makes sense.

No. Its the flow rate out of the injectors in to cylinders.

so if I was to do that then...

0.3 MPG = 55 MPH / 150 GPH Lift Pump

As you can see the math doesn't work out at all...

Here is a live display of my ScanGauge and its calibrated for my truck...

http-~~-//www.youtube.com/watch?v=SRQdoPs_tLo

Now you'll see the math in real time... My offset in the SGII are 22.5% GPH offset and 1% offset for speed.

It's the fuel the ECM thinks is going out the injector, the problem is that it doesn't think very well!!

As you can see from AH64ID post his offset wonders all over the map. :rolleyes:Mine is constant right at 22.5% from the GPH pulled from the OBDII port. Now a bit more on this offset changes with fuel system mods. Like stock injectors 235 HP is 19.6% and now with RV275 Injectors its 22.5%Then with my 265/75 R16 tires is 0% offset but with the 235/85 R16 tires I've got a 1% offset. It's the fact there is roughly 1/2 MPH off so I bump the offset to make my speed lower pulling my distance shorter and MPG a taste lower but at least it very very close. Now with any generation of overhead console computer they are all factory fixed with particular values and can't be changed so as we modify our truck the overhead computer get nicknamed "Overhead Idiot!"...

For some reason the 03-07 5.9s are all over the board like I demonstrated, but the pre 03 seem to be consistent and the 07.5+ seem to be quite accurate. Look at my annual averages, it looks like Dodge expects the truck to get more efficient as the miles add up, but it doesn't do it quite as much as they hope. So far this year I have varied from 8.39% to 20.21% off, really hard to use accurately. Thou the annual average is about 17.5% since 2009. 2007: 3.64% 1831 miles tracked2008: 10.95% 14650 miles tracked2009: 17.46% 22341 miles tracked2010: 17.5% 13210 miles tracked2011: 18.0% 9717 miles tracked2012: 16.1% 3160 miles tracked so far

I think the easiest solution is to have a truck with no computer in it like mine. The only way to calculate it is with pencil and paper. This way is mostly flwaless, but i dont ever seem to be able to refill at the same station, same pump, same direction as recomended. just my .I still run the tire size that came on the truck, 265 75r 16's.