Gage vs. Gauge
Gauge is a word that is used to refer to measuring instruments that measure a physical characteristic of an object. Gage is a word that has two meanings, one of which is pledge. This means it is used to refer to a surety or an item that is deposited in lieu of a loan. The word has the same root as we find in engage and wage. Engage reflects one’s promise to marry or the willingness to enter an agreement. The root also finds use in mortgage.

Key Differences
As verbs the difference between gage and gauge is that gage is (obsolete) to give or deposit as a pledge or security; to pawn or gage can be (gauge) to measure while gauge is to measure or determine with a gauge; to measure the capacity.
As nouns the difference between gage and gauge is that gage is something, such as a glove or other pledge, thrown down as a challenge to combat (now usually figurative) or gage can be a variety of plum while gauge is a measure; a standard of measure; an instrument to determine dimensions, distance, or capacity; a standard.
Gauge is used more in conjunction with a device that has moving parts, and gage seems to be used more with devices that don't have moving parts, i.e., jo-blocks, ring or plug thread gages, etc.
A stream gage is different than an earth-pressure gauge in functioning.

Aimie Carlson
Nov 24, 2015
When the unit of measure is taken by direct reckoning, use gage. When there's a dial or something else that moves in response to external force (or stress) use gauge.

Aimie Carlson
Nov 24, 2015
Though there are people who use the spelling gage for a measuring instrument, the correct word is gauge that is used both as a noun as well as verb.

Aimie Carlson
Nov 24, 2015
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There are other meanings of gage, and it can be a spelling variant of gauge, as well.
It appears that "gage" has become a colloquialism for "gauge", presumably because of laziness & poor spelling. In reality, they both have clear different meanings in terms of definition and function.

Aimie Carlson
Nov 24, 2015
Gage is a root in many English words such as engage, wage, and mortgage.
Gage and Gauge Definitions
Gage
Something deposited or given as security against an obligation; a pledge.
Gauge
The distance between the two rails of a railroad.
Gage
Something, such as a glove, that is offered or thrown down as a pledge or challenge to fight.
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Gauge
The distance between two wheels on an axle.
Gage
A challenge.
Gauge
The interior diameter of a shotgun barrel as determined by the number of lead balls of a size exactly fitting the barrel that are required to make one pound. Often used in combination
A 12-gauge shotgun.
Gage
Any of several varieties of plum, such as the greengage.
Gauge
The thickness or diameter of sheet metal, wire, or a similar manufactured material or piece.
Gage
Variant of gauge.
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Gauge
The fineness of knitted cloth as measured by the number of stitches per a given unit of length.
Gage
To pledge as security.
Gauge
A standard or scale of measurement
The capacity of barrels was measured according to the gauge in use at the time.
Gage
To offer as a stake in a bet; wager.
Gauge
An instrument for measuring the dimensions, capacity, or amount of something
A pressure gauge.
A fuel gauge.
Gage
To bind by pledge, or security; to engage.
Gauge
A means of estimating or evaluating; a test
A gauge of character.
Gage
(archaic) To wager, to bet.
Gauge
(Nautical) The position of a vessel in relation to another vessel and the wind.
Gage
(obsolete) To give or deposit as a pledge or security; to pawn.
Gauge
To measure the dimensions, capacity, proportions, or amount of (something), especially by means of a gauge
Gauged the thickness of the metal part.
Gage
(US) gauge {{gloss}}
Gauge
To evaluate or estimate
Gauge a person's interest.
Gage
Something, such as a glove or other pledge, thrown down as a challenge to combat (now usually figurative).
Gauge
To adapt or make conform to a specified standard
Pressure valves that are gauged to industry requirements.
Gage
(obsolete) Something valuable deposited as a guarantee or pledge; security, ransom.
Gauge
To chip or rub (bricks or stones) to size.
Gage
(US) gauge {{gloss}}
Gauge
To stretch (a pierced earlobe or other body part) by inserting progressively larger objects over a long period of time.
Gage
A subspecies of plum, Prunus domestica subsp. italica.
Gauge
Of or relating to a gauge.
Gage
A pledge or pawn; something laid down or given as a security for the performance of some act by the person depositing it, and forfeited by nonperformance; security.
Nor without gages to the needy lend.
Gauge
(Physics) Invariant under a local transformation.
Gage
A glove, cap, or the like, cast on the ground as a challenge to combat, and to be taken up by the accepter of the challenge; a challenge; a defiance.
Gauge
A measure; a standard of measure; an instrument to determine dimensions, distance, or capacity; a standard
Gage
A variety of plum; as, the greengage; also, the blue gage, frost gage, golden gage, etc., having more or less likeness to the greengage. See Greengage.
Gauge
An act of measuring.
Gage
To give or deposit as a pledge or security for some act; to wage or wager; to pawn or pledge.
A moiety competentWas gaged by our king.
Gauge
An estimate.
Gage
To bind by pledge, or security; to engage.
Great debtsWherein my time, sometimes too prodigal,Hath left me gaged.
Gauge
Any instrument for ascertaining or regulating the level, state, dimensions or forms of things
Gage
Street names for marijuana
Gauge
A thickness of sheet metal or wire designated by any of several numbering schemes.
Gage
A measuring instrument for measuring and indicating a quantity such as the thickness of wire or the amount of rain etc.
Gauge
(rail) track gauge
Gage
Place a bet on;
Which horse are you backing?
I'm betting on the new horse
Gauge
(rail) loading gauge
Gauge
A semi-norm; a function that assigns a non-negative size to all vectors in a vector space.
Gauge
(knitting) The number of stitches per inch, centimetre, or other unit of distance.
Gauge
(nautical) Relative positions of two or more vessels with reference to the wind.
A vessel has the weather gauge of another when on the windward side of it, and the lee gauge when on the lee side of it.
Gauge
(nautical) The depth to which a vessel sinks in the water.
Gauge
(plastering) The quantity of plaster of Paris used with common plaster to make it set more quickly.
Gauge
That part of a shingle, slate, or tile, which is exposed to the weather, when laid; also, one course of such shingles, slates, or tiles.
Gauge
(firearms) A unit of measurement which describes how many spheres of bore diameter of a shotgun can be had from one pound of lead; 12 gauge is roughly equivalent to .75 caliber.
Gauge
A shotgun (synecdoche for 12 gauge shotgun, the most common chambering for combat and hunting shotguns).
Gauge
A tunnel-like ear piercing consisting of a hollow ring embedded in the lobe.
Gauge
Cannabis.
Gauge
(transitive) To measure or determine with a gauge; to measure the capacity of.
Gauge
(transitive) To estimate.
Gauge
(transitive) To appraise the character or ability of; to judge of.
Gauge
To draw into equidistant gathers by running a thread through it.
Gauge
(transitive) To mix (a quantity of ordinary plaster) with a quantity of plaster of Paris.
Gauge
(transitive) To chip, hew or polish (stones, bricks, etc) to a standard size and/or shape.
Gauge
To measure or determine with a gauge.
Gauge
To measure or to ascertain the contents or the capacity of, as of a pipe, barrel, or keg.
Gauge
To measure the dimensions of, or to test the accuracy of the form of, as of a part of a gunlock.
The vanes nicely gauged on each side.
Gauge
To draw into equidistant gathers by running a thread through it, as cloth or a garment.
Gauge
To measure the capacity, character, or ability of; to estimate; to judge of.
You shall not gauge meBy what we do to-night.
Gauge
A measure; a standard of measure; an instrument to determine dimensions, distance, or capacity; a standard.
This plate must be a gauge to file your worm and groove to equal breadth by.
There is not in our hands any fixed gauge of minds.
Gauge
Measure; dimensions; estimate.
The gauge and dimensions of misery, depression, and contempt.
Gauge
Any instrument for ascertaining or regulating the dimensions or forms of things; a templet or template; as, a button maker's gauge.
Gauge
Any instrument or apparatus for measuring the state of a phenomenon, or for ascertaining its numerical elements at any moment; - usually applied to some particular instrument; as, a rain gauge; a steam gauge.
Gauge
Relative positions of two or more vessels with reference to the wind; as, a vessel has the weather gauge of another when on the windward side of it, and the lee gauge when on the lee side of it.
Gauge
The distance between the rails of a railway.
Gauge
The quantity of plaster of Paris used with common plaster to accelerate its setting.
Gauge
That part of a shingle, slate, or tile, which is exposed to the weather, when laid; also, one course of such shingles, slates, or tiles.
Gauge
A measuring instrument for measuring and indicating a quantity such as the thickness of wire or the amount of rain etc.
Gauge
Accepted or approved instance or example of a quantity or quality against which others are judged or measured or compared
Gauge
The distance between the rails of a railway or between the wheels of a train
Gauge
The thickness of wire
Gauge
Diameter of a tube or gun barrel
Gauge
Judge tentatively or form an estimate of (quantities or time);
I estimate this chicken to weigh three pounds
Gauge
Rub to a uniform size;
Gauge bricks
Gauge
Determine the capacity, volume, or contents of by measurement and calculation;
Gauge the wine barrels
Gauge
Measure precisely and against a standard;
The wire is gauged
Gauge
Adapt to a specified measurement;
Gauge the instruments
Gauge
Mix in specific proportions;
Gauge plaster
What is Gage?
Gage is also referred as gauge at some places with the definition of a device used for measurement. Gage is something, such as a glove or other pledge, thrown down as a challenge to combat (now usually figurative) or gage can be a variety of plum. Gage is (obsolete) to give or deposit as a pledge or security; to pawn or gage can be (gauge) to measure
What is Gauge?
A gauge can be described as “a device for measuring a physical quantity”, for example “to determine thickness, gap in space, diameter of materials, or pressure of flow”, or “a device that displays the measurement of a monitored system by the use of a needle or pointer that moves along a calibrated scale”.