So I ordered a KR scan gauge today .

I've had mine for 4 years. It shows boost, HP, oil pressure, voltage.
I find it especially useful for controlling the boost with conjunction of HP while climbing up hills. That's where I control the engine stress and mpg. Other than that, the oil pressure gauge is of great help, as that little analog clock on Volvo dash is to small to show anything meaningful. I wish there were more parameters read by it. Especially, oil temperature which is available but the scan gauge does not have it on its display.

Other features I find difficult to work with. I never had enough patience to figure out how to use the "as you go" profit...I tried but with no success. Maybe that's because the tank fill up feature was never accurate and I gave up on it.

I also had to work on Keep Trucking ECM "Y" splitter to have them both connected. One of the prongs was too thick and I had to file it down.
Here's the link to the thread where I found solution that worked for me.
KR scanguage and Keep Truckin Y splitter for Volvo.

 

Advance auto sells them and get 20% off.

 

i sold my truck with scangauge. Purchased new truck and new scangsuge D. This scangauge D came in without plastic packaging
I bought it from manufacturer's webpage. looks like there are few more settings related to display in this unit.

 

Scangauge is good one but Murphy Powerviue is more advanced...