....on my Faria Gauge keep frustrating me..most speedos tend to exaggerate but mine shows at least 3-5 mph less than on GPS....It is the white faced Faria Gauge (0-80mph)that works the conventional way (pick up port on transom connected to the gauge by a pressure hose) The position for the pick up is perfect and the hose is in good condition.
Is there any way of adjusting the reading by modifying the pick up or the hose to get the reading up a little? I want my speed back!!
Any suggestions welcome, thanx guys!
By the way: Boat is a 22" Pachanga with 383 SBC.
A buddy of mine has the same problem, with a 27" Chris Craft Concept........

Check the pitot for obstructions.
Remove the hose from the speedo end and back blow it to see if it's clear.
Someone posted a while back the air pressure psi vs mph equivelant for speedo calibration. Check back of gage for adjustment screw/port.

Note: On most mechanical gages the first and last 10% of gage scale are generally inaccurate. Any mechanical gage is most accurate in the middle 60% of scale. First 10% reads high and last 10% reads low.
Example: Speedo reads 5 mph to 80 mph (75 mph scale).
You should use the readings from 5 to 13 mph (lower 10%) and 67 to 80 mph (upper 10%) for reference only.
Most accurate readings should be from 26 to 63 mph (middle 60%).

Best bet is to switch to a GPS.

I have a similar problem the speedo reads about 3 less than the GPS. different pickups will change the reading. I think that if you can angle the pickup openinga little tward the keel the reading will increase a little..

That would make sence.
Think about it, the water is trying to flow diagonally up from the keel as the boat travels forward at speed.
But would only be true very close to the hull surface. If the pitot was two to three inches or so below the hull it shouldn't make any difference.

Maybe your pick-up is starting lose contact with "clean" water as your hull comes up out of the water with more speed. Just a thought.