I've never used a SGII, but I did try using a PC-based software, OBDwiz, on my diesel truck. Here's what I found out. The fuel flow for all these devices (SGII, Ultragauge, Torque, etc.) is based on messages available over the OBD port and can be done three different ways:
1) Intake manifold pressure (MAP) based
2) Mass air flow (MAF) based
3) Direct method
Methods 1 & 2 assume that the air-fuel ratio is constant. That's a reasonable assumtion on a gas engine, but it widly inaccurate on a diesel. You might be able to fudge the adjustment factors so that you'll come in close, but it'll never tell you the real truth. The AFR at idle can easily be several times higher than it is at high loads--that leads to large errors.
Method 3 only works IF fuel flow is broadcast over the OBD port. That fuel flow info is based on the injector fuel maps and should be reasonably accurate. Unfortunately, not all diesel report that info over the OBD port. Unfortunately, my truck doesn't
so I gave up.
In my opinion, the direct method is the only one worth anything on a diesel.
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