Fuel Economy: Garmin vs Yamaha

Ever since we have had our boat (R23), the Garmin display and the Yamaha display have never agreed on the Economy. The Garmin always reports a (sometimes much) lower number, even when the Garmin is displaying the Yamaha screen. (All other numbers including speed/gph/etc are the same). For example, here is a picture where the Yamaha unit shows 5.5mpg but the Yamaha screen in the Garmin shows 4.3mpg.



I wasnt sure which one was actually right, but the Yamaha unit *felt* like it was right. Yesterday I snapped this picture where the Yamaha screen on the Garmin said 0mpg while the Yamaha unit said 2.1mpg:



I know for sure I was not getting 0mpg. So my questions are:

1. Why would the Yamaha screen on the Garmin show a different number?
2. How can I fix it?

Am I to assume your info is coming from the NMEA 2000? I am just guessing here, but perhaps the Yamaha is getting the calculations from a different source than the Garmin. It does not make sense, but I cannot think of another reason as to why the numbers are different. I used to use two different tire gauges to read my tire pressure and always had differences. Then my friend, an engineer, told me to just use one since there can always be differences between instruments. I assume the Yamaha is using an internal GPS to make its calculations, since you can get that info even if not connected to the network. And perhaps the Garmin is getting the fuel flow from the same point, but the GPS in the Garmin is showing different information than the Yamaha. However, your speeds and fuel flow are the same on each screen.

Your numbers seem a little 'extreme' for this solution... But my Garmin has (for a completely unknown reason to me) a place to "Offset" the Yamaha input by a percentage. Again, no idea why, but it's there.

Is it possible that the offset in your Garmin is so high that it gives these numbers?

Problem is that I had to hunt to find where to set the offset to "0"... and for the life of me, cannot remember where that was.

Here's hoping you find an easy fix! 😀

Yes, I've found the same thing. The Garmin MPG is mostly inaccurate, and almost always lower than the actual MPG. Why I don't know, However, I've found that the fuel used, as reported by the Garmin is accurate (to within 1%) and the GPH reported on either Garmin or Yamaha displays is also accurate (and stable for a given RPM), so I've resorted to using my calculator to divide the speed by the GPH to get a value for MPG.

A thought.

Does Yamaha measure distance through the water using the pitot tube it uses for measuring speed? Garmin will be measuring speed and distance over the ground. Given some of the currents around here that would account for some discrepancy. (Not 0 mp obviously).

The Yamaha unit is set to use GPS as the source for speed. Even if it was not, currently both displays show the same speed at all times (even when mpg is wrong). The weird part is that they both show the same speed, and same gph, so the mpg *should* work out the same due to simple math. But they dont. On the Yamaha display, mpg divided by gph shows the same mpg. But on the Garmin, even though mph and gph are the same, the mpg is different and no calculation I can come up with would result in that number.