MPG calibration

1pointofview

Is there a way to calibrate the MPG gauge?  my MPG gauge has never really worked the way I think it should.  I will be doing 32-34 mpg via GPS and will be going 15 based on the MPG gauge.  Any way to tweak that?

nquirk

@1pointofview.  Just to clarify, I believe you mean MPH (miles per hour) not MPG (miles per gallon)  I ask, as I have both so I want to clarify.  Since you've mentioned GPS, I'll assume MPH and your speedometer.

The speedometer is attached to the back of your boat and submerged in water, it is actually an air pressure gauge. Water jams into the "pitot" as speed increases which pushes on the air in the tube causing a corresponding rise in air pressure.  This is the typical boat mounted version that would be hanging off of your transom (see picture below) They usually do not read until 5 mph

However, if your speedometer is telling you 15mph, and your GPS says 32-34mph, you could have a faulty speedometer, or the pitot tube could be clogged or kinked.

I suggest you start with the cheaper solution (free) which is trace the hose and look for kinks.  If there are not, then remove the end of the tube and use compressed air to try to see if there is a blockage/debris in it.

 

NOTE:  DO NOT USE excessive amounts of air.  Use a compressor with a regulator or can damage/explode the hose and/or damage the gauge if you blow back that way.  Disconnect hose at both ends, and blow it out with low pressure (perhaps 10psi or so) in short bursts. 

Now to address the question on calibration.  If these are working correctly, I'm not aware of an adjustments to calibrate however I believe Teleflex makes a more accurate version of this.

Next to that, is the type that are paddle wheel based and more accurate.  I have one of these on mine and works more closely to GPS.

And, on the MPG gauge, I have a Smart Monitor for my Mercury outboard that reads direct from the engine.  I monitors fuel flow in GPH and converts it to estimate range based on fuel flow.

 

Hope this helps.