I've been using a Scangauge II on my vehicles for years and have used it on my G35x since I bought it in November. It usually takes me a gallon and a quarter to get to work and my idle consumption is typically around 0.36-0.40 GPH. For some reason today on my drive to work and my drive home my consumption was right around a gallon and a half and my idle is sitting at about 0.50 GPH. What could cause this increase? Idle RPM is sitting right around 625-675, no codes popping up, no unusual sounds.

 

If you're going off of the fuel level sensor data you're doing it wrong. The only true way to.calculate mileage is by dividing the ACTUAL FUEL GALLONS by miles travelled.

Especially on a vehicle that's notorious for having the fuel gauge level sensor go out.

Track it manually for a while and see if it's actually getting poor economy.

It's also springtime which is a great time for tune ups.

 

That's what the scangauge is for, gets calibrated at each fillup, never more than 1 or 2 tenths off at each fillup. It gives the GPH based on info it pulls from the OBD II port.

 

 

I'd say it needs to be recalibrated and run a few times to determine accuracy against actual math.

There is no GPH value for any OBD2 protocols so the scangauge is using an internal algorithm based on odometer and fuel level with some kind of math to account for maybe throttle position or rpm or something.

I've never used one so I don't really know what needs to be done to "fix" it but you can easily verify actual fuel economy with a calculator, your trip odometer, and the exact gallons needed to fill the tank.