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  Discrepancy in SmartCraft Fuel Usage Readings

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Author Topic:   Discrepancy in SmartCraft Fuel Usage Readings
Frank O posted 03-24-2012 04:39 PM ET (US)   Profile for Frank O   Send Email to Frank O  
When I repowered my 1995 Outrage 21 with a 2011 OptiMax 250 this winter, I went with a SmartCraft instrument package consisting of a speedometer, tachometer, and five dedicated instruments. Both the speedometer and tachometer can be set to display fuel used.

Today, my son and I took the boat out for a relatively short 25-minute run. Right before starting out, we reset the fuel-used display to zero on both the speedometer and tach. When we returned, the tach said we had used 0.4 gallon, while the speedometer said it was 0.2 gallon.

Any idea why this discrepancy would exist? Do the two instruments calculate fuel usage in different ways? During the sea trial when I took delivery of the outboard, the rigger did a little tweaking to the instruments, but I'd be surprised if it could have caused this much of a difference. (Yes, I could run the boat longer to see how much discrepancy persists, but since we're running on a portable tank for a bit while working on the built-in tank, I don't want to push my luck.)

jimh posted 03-25-2012 08:27 AM ET (US)     Profile for jimh  Send Email to jimh     
Typically a modern engine like your OptiMax will report the fuel flow rate. The fuel flow rate is accumulated by an external instrument or gage, which totalizes the amount of fuel used. It is also possible that fuel flow could be measured by an in-line fuel flow sensor.

If both the speedometer gauge and the tachometer gauge can display fuel usage, and if they display different readings, there are several possible causes for this that occur to me.

First, the two gauges could be set to measure different units. Make certain both gauges are configured to show the fuel volume in the same unit, which I imagine would be US Gallons.

Second, if the gauges are independently calculating and totalizing the fuel used, then each gauge would need to be reset. If you only reset the tachometer fuel usage data, for example, it might not be reset on the speedometer, if the speedometer is calculating its own fuel usage.

Third, if the gauges are independently calculating the fuel usage, there may be a calibration setting to improve their accuracy. The calibration could be different.

If only one of the gauges is actually calculating the fuel volume used, and the other gauges just repeat that value, the data should be the same on all gauges, unless the preference for units is set differently on one of the gauges.

RobP posted 03-27-2012 05:54 AM ET (US)     Profile for RobP  Send Email to RobP     
I typically get 0.1 or so difference between the two gauges. I don't worry too much about that little bit of difference, since it stay consistent over a tank.

What I would be more interested in is how you ran a 250 for 25 minutes and only burned 0.4 gal of fuel. Since you are running on a portable tank that is a perfect chance to check if your fuel rate is being detected correctly.

Frank O posted 04-03-2012 08:49 PM ET (US)     Profile for Frank O  Send Email to Frank O     
Yes, next time I run on the portable tank I'll measure the actual amount of gas used.

Most of the 25 minutes was running from the slip to the harbor entrance and back, with just a few minutes outside the harbor at higher speed. At 5 mph the gauges indicated that the instantaneous gas consumption was very low.

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