Scan Gauge II question
 

I got my Scan Gauge II on Saturday! Thanks for the great price Ecomodder!

My question is this: periodically, the instant MPG setting will go to 9999 and take several seconds to return back to a realistic MPG number. Is this a normal activity?

That's where you want it. That means that you aren't using any fuel. You'll find that it happens when you're decelerating with the RPMs starting over 2000. Your car is going into Deceleration Fuel Cut Off (DFCO). Here in Colorado, it's not unusual for me to hold my car back on extended descents for miles in DFCO, but it's normal in day to day traffic, too.

Cool.
2nd question...

Truck is 97 Ram. Truck was originally a 5.2, but it died. I bought what was supposed to be another 5.2, ended up as 5.9/360cu in.

Nothing else was changed. Same injectors, computer, etc.

Would I load the Scan Guage as 5.2 or 5.9?

5.9, but realize this is going to give you estimated mpg, not super accurate data until you calibrate it with your second fill up.

The first fill up, go to menu, fill up, and save without adjusting the gallons filled. That let's the SG know you filled. The next tank do adjust the gallons used and save it and your data will be much more accurate in the future.

Even though the computer and injectors think it is the 5.2? How does the ScanGauge come up with the MPG numbers in the first place? I presume it is injector duty cycle related, but if the truck thinks it has a 5.2, would the SG not also need to match, even though it had a larger engine?

I'm not certain but it won't matter after you calibrate it. Either way, this first tank is just a guess for the device anyway. I didn't even bother to set my engine size initially and defaulted to 3.0L. That made me drive even more conservatively than I would have, looking at sub-30 mpg numbers all the time. Now, properly set up and calibrated, I get light footed when I'm under 40.

In any case, it all works the same. Now you have a number to watch besides the speedo and that alone changes the way you drive. Instead of the single most important thing being when you arrive, it becomes mpg. Just that simple change in motivation usually equals 20% or better.