How accurate is a Scangauge 2 for MPG?
 

My Scangauge 2 shows that my dark blue 2005 Civic gets reduced mileage when going up hills or accelerating, and increased mileage when going down hill or stopping. That makes sense. What does NOT make sense is that the MPG is highest at around 60-70 MPH. Should I just drive the speed that it reports the best mileage where legal and safe? A friend said that I should believe my Scangauge because my car will get its best mileage at higher speeds because, unlike his truck, my car is aerodynamic, so it is more efficient at a higher speed. That sort of makes sense. Is he right? Thanks, I want to save fuel by driving fast if possible:)

It's accurate once it's calibrated, but it's relatively accurate even before that- higher numbers, whether or not they're "real" are higher than lower numbers.

Gearing can leave the engine at its most efficient spot, and steady cruising doesn't put too much load on an engine, especially on something aerodynamic. My Fit does best between 40 an 50 mph because it's a brick.

Calibrate it at every fill up for a while, and the numbers will get more accurate and you'll be able to believe the screen a lot more.

Generally, on a level road your car will get its best mileage when it shifts into top gear.

(Is yours manual or automatic? Make a garage entry so your vehicle details are accessible under your username!)

The instantaneous readings are accurate once you've got it calibrated to your car.

Thanks a lot! It has a 4 speed automatic transmission. Also, how do I calibrate my Scangauge? I bought it used and it did not come with the manual.

When you fill, you go something like:

MORE > FILL UP

And it gives you the chance to enter or correct the volume of fuel you actually purchased in place of the volume of fuel the SG calculated you burned. You're adjusting a "fuel offset" which is applied to further calculations.

Note: You can also adjust a speed/odometer offset.

I'm sure you can still get the user manual from their site. Yup:

https://www.scangauge.com/support/user-manuals/

My experience, comparing it against the Insight's built-in gauge, is that it's not even close.

But which one is right?

That's mainly because the ScanGauge isn't designed for lean burn vehicles. It assumes 14.7 to 1 air/fuel ratio.

I wish I could get ethanol free fuel in Ohio. In North Carolina, I used to get ethanol free fuel from a Shell near work, but I can't find ethanol free fuel.

If looking for Ethanol free fuel in the states, try this site:

https://www.pure-gas.org/