Yes, the computer knows what size everything is, but the SG is trying to calculate fuel usage based on the live sensor data it reads. Knowing what size the engine is, how large the fuel tank is, and how many gallons are put in the tank at each fillup helps it calibrate.
Yes, I switched the SG between my 2 vehicles and forgot to reset and it seemed off a little......then I realized and reset it.
Jay has you headed in the right direction, but let me draw a parallel so maybe this will all make more sense.
Originally Posted by vectorg
Are you saying my car's computer doesn't even know the displacement of it's own engine?
It's not about your ECC, it's about your Scangauge. The ECC isn't calculating your fuel efficiency and just handing the numbers over; it's busy managing the engine. Your Scangauge is doing that job based on speed, injector pulse width, and similar things.
Here's the parallel.
When you buy a new TV and hook it up to your cable, does it automagically know what channels are available? Nope. And the cable company doesn't send any sort of message that tells it, either.
So before you use your new cable-ready TV, you go into the setup menu and tell it to scan for available channels. It stops briefly on every possible channel to see if there's a signal. If it finds one, it marks that as a working channel, otherwise it marks it to be skipped in the future.
Similarly, the Scangauge doesn't know anything about the vehicle/engine/etc. to which its connected.
So like the new TV, before you can use your Scangauge effectively you have to tell it certain things about the car you've hooked it to, like the size of the gas tank and the engine displacement.
Displacement will get you into the right range for fuel efficiency, but be aware that your Scangauge is calculating (estimating) your fuel usage. It can make really, really good estimates, but the default displacement setting might be off a little for your particular engine (maybe one of the injectors is a little clogged or something; who knows?) That's what the tank refill adjustment is for. When you refill, you tell the Scangauge how many gallons you put in the tank. It then compares what the car really used with what it thought should have been used, and works out a correction percentage. Usually a single tank will do it; if not, the second surely will. After that you'll just bounce around due to slightly different fill levels and other variables.
Once you have the adjustment factor set (or really close), you don't have to mess with it again. Just do a quick sanity check to make sure the refill is close to what the Scangauge was expecting, then hit the "fillup" button to tell it you're starting a fresh tank.
Your Scangauge is doing that job based on speed, injector pulse width, and similar things.
Actually, the ScanGauge doesn't have access to injector pulse width or duty cycle, which is not part of OBDII data. It estimates based on the MAF and O2 data, which tells it how much air goes in and what the A/F ratio is.
That's another reason you have to fine-tune it manually.
Thanks, Rick, that was a good explanation. Changing from 3.0 to 2.2 liters made a big difference in my mileage reading. But now I understand that I need to enter some more info when I do my fillups. Got it.