Scanguage says 25mpg, but manual measurement says = 35mpg
 

If I measure from top of tank to top of tank, plug in the cost and the distance I travel I get 35mpg for my van.

However, if I plug in my scangauge i get 25mpg max... I never see it rise to 35...
I have the wheel diameter set correctly and the same engine size (2,5 turbo...)

You need to calibrate your Scan Guage. Next time you fill up you can enter the amount of gallons/liters used into the SGII to calibrate it.

yep this is what you need to do. Do it for two tanks and it should be very close.

It will never be right on but will stay within a % or 2.

I disagree: the Scangauge just isn't very good at measuring fuel consumption. I have one in my Insight, which also has the factory mpg gauge. The factory one is as close to accurate as I can measure, the Scangauge reading often differs by 20 mpg or more. That's when it's calibrated, and it loses calibration very easily.

The Insight has weird A/F ratios (up to 1:25 or something) and the SGII was designed for 1:14.7. So it won't be accurate in less you don't drive in lean burn mode.

Mine is consistently accurate within 1% once calibrated.

...my SGII, on my SIDI '09 Vibe, seems to run within ±2% fairly consistantly, and that's with the Toyota Spark-Ignition Direct Injection (SIDI), not normal MFI.

jago25_98 -

I would venture to guess that the SG is not as good at turbo engines.

Here's a silly question but I have to ask it. Is the fuel set to US gallons or Imperial gallons?

CarloSW2

US gallons

Yeah, that's the problem (at least as I understand it): the SGII isn't actually measuring fuel consumption directly, it's inferring it from other measurements and some assumptions, like the A/F ratio. If your engine doesn't follow the assumptions it uses, the values will be wrong.

There are other cars besides the Insight that have some variety of lean burn tech: Lean burn - Wikipedia, the free encyclopedia