Out of the blue here where the weather has been in mid 20s at night and mid 40s during the day with high winds for weeks
The weather went wacko, as the normal here :-(

Three days of 60s at night and 80s during the day, going crap again cold this weekend
so was a good time while all the nuts were on the road for holidays to do a few things with my 99 C5 and checking the torque on the lug nuts of wheels, just did not feel or seem right
Started thinking and recalled I had not calibrated the torque wrench in some years :-(

So I had to go find old info I saved back than in process of calibrating torque wrenches by hand

Lots of photos below as I tested 2 torque wrenches that report in ft/lbs
One is smaller and output is in inches/Lb
and 1 that is digital

In summary
as my photo shows, measure the handle, write the distance down
Take anything that has weight, I used a gas can and filled with water that total weighed 25 pounds
Take rope and place it to your measured mark of TW handle,
and the other end of rope so that the weight would hang down but not touch the ground

Simple math, shown below but
if the length mark of TW is 15 inches multiply that with the 25 pounds of weight and to get answer in feet divide then by 12
( do not do this if wanting inch/lbs answer

So 14.5 inches *25 pounds= 362 / 12 = 30 ft/lbs

Now set the TW at 30 ft/lbs
Remove a box or whatever you used to hold your weight off the ground
so now that weight dangles above the floor and the weight is all on the end of TW handle
When doing that, you should hear the wrench click
IF not that use the adjuster of wrench and keep testing till the click is correct for the weight and the setting

To assure all is calculated, set wrench below 30 ft/lb ( or what ever your numbers come to)
test and then set wrench to above your math numbers and assure no click is heard

Was it worth it ?, been a long way around on this,
but YES

The TW I was using to torque the lug nuts was way off and trying to calibrate it failed
so over years of doing a NO NO of leaving tool to a set ft/lb caused the internal spring to screw up and is now
just a ratchet socket wrench

The others I tested all calibrated correctly

With GM vehicles over the last 20 plus years using a onetime use, Torque to Yield
you really want to be sure you're installing them to GM specs of foot/pounds (or inch/pounds)

Here is the science of all this and detail :

Hand Calibrate Torque Wrench
Measure from the square drive to the handle.
The square drive is the end of the torque wrench you’d attach a socket to.
For the sake of simplicity, use whole inches rather than using any fractions.
Mark the point you measured to on the handle and record the distance on a piece of paper for you to come back to later.
• Set the paper aside in a safe space until you need it.
• Because 24 inches (61 cm) is a common length for most torque wrenches, it will serve as the measurement for further steps.
Secure the square drive in the vice.

Orient your bench vice so you can place the square drive of the torque wrench into and have the handle extend out, away from the table or bench. Then insert the square drive into the vice and tighten it until it’s secure.
• Be careful not to over tighten the vice and damage the square drive on the torque wrench.
• Make sure only the square drive itself is caught in the clamp, so the wrench can move under the weight you apply.

Calculate the appropriate setting for your weight.

The equation is: handle distance times weight divided by 12. In order to determine the right setting for the torque wrench, multiply the distance you measured in step 2 by the 20 pounds you will be using for your weight.
That comes out to 480 inch-pounds (24 inches times 20 pounds) which equates to 40 foot-pounds (480 inch-pounds divided by 12).

• If you're working with metric units, start by converting the weight to Newtons. To do this, multiply the number of kilograms by 9.807. In this example 9.07 kg x 9.807 = 88.94949 Newton.
• Then, multiply the number of Newtons by the length in meters: 88.94949 Newtons x 0.6096 meters = 54.2 Newton meters.
• To convert foot-pounds to Newton meters, multiply by 1.35582. For this example, 40 foot-pounds is equal to 54.2 Newton meters.
• Be sure to use the correct distance and weight figures. If your wrench is a different size or you’re using different weights, your figures will be different.

Hang the weights from the handle of the wrench.

Tie the rope to the weight and make a loop that you can hang from the handle of the torque wrench where you made your mark in step 1.
Make sure the length of rope is short enough that the weight won’t touch the ground once you hang it.
• Do not securely tie the weight to the wrench. Instead, simply hang it.
• Be sure nothing is in the way of or supporting the weight as it hangs.

Adjust the torque wrench using the weight.

You can usually adjust the spring tension in a torque wrench by turning a screw located midway up the wrench’s handle with a screwdriver. Hang the 20 lb (9.1 kg) weight from the torque wrench at your first mark and see if it clicks. If it doesn’t, tighten the spring by turning the screw clockwise, then lift the weight and lower it again to test it.

• Repeat this process until the torque wrench clicks using the known weight.
• Be sure to lift the weight off the wrench and lower it again to test for clicking each time.
Move the weights up the handle if you hear a click.
Listen for a click from the torque wrench as you hang the weight from the marked point on the handle. If you hear one, lift the weight off the handle and set it down again further up the neck, moving toward the head of the wrench.
• Keep repeating this process until you stop hearing the click.
• Be sure to lift the weight off and set it down again each time. Do not slide it up the handle.
Lower the weights if you don’t hear a click. If you don’t hear a click from the torque wrench as you place the weight on it, move the weight down the handle of the wrench until you do hear one.
• Start by moving the weight an inch or so at a time.
• It’s okay to move up and down the handle of the wrench more than once as you look for the point in which it starts to click

Mark the point of transition.

Once you find the point in which the handle transitions from clicking to not, mark it on the wrench with your pen. Be sure to mark the point exactly, so you may need to make multiple attempts at identifying where it is by moving the weight up and down the handle.
• The part of the handle where it begins or stops clicking is called the transition point.

Measure from the square drive to the point of transition. Use the measuring tape to find the distance from the square drive to the transition point you identified using the weight. Record that number on a piece of paper and set it aside. A measurement of 26 inches (66 cm) will be used for this example, but yours may be different.
• Be careful not to confuse this number with the figure you recorded in step 2.
• You may want to test finding the transition point more than once to be sure you have the correct number.
Calculate the applied torque.
If the transition point of the torque wrench with 20 pounds was at 26 inches, for instance, multiply that by 20 pounds to determine the amount of torque actually applied: so 26 inches times 20 pounds equals 520 inch-pounds, or 43.33 foot-pounds (520 divided by 12 inches ).
• The equation is the same as before: measurement length times weight, divided by 12.
• If you're using metric units, convert the weight to Newtons (kg x 9.807), then multiply the number of Newtons by the length in meters: 9.07 kg x 9.807 = 88.95 Newtons. 88.95 Newtons x 0.6604 meters = 58.74 Newton meters.

Correct for the difference you identify.

If you are unable to adjust the torque wrench, you can still use it pretty accurately if you adjust the settings you use on the wrench to compensate for the difference. Divide your first measurement by the transition point (in this case, 24 divided by 26, which equals 0.923). Whenever you need to use the torque wrench, multiply the correct torque by this number
• Multiplying your intended torque by the difference will give you the correct setting for your specific torque wrench.
• This solution can keep you working, but the wrench will still need to be calibrated.

Other ways :

The Fix: Math vs. Mechanical Adjustment
So your wrench didn’t click. You have two options.
Option A: The “Correction Factor” (The Safe Way)
If your wrench didn’t click at 30 ft-lbs, dial the setting down to 29, then 28, then 27 until it finally clicks with the 20lb weight.

• Let’s say it clicked at 27 ft-lbs (but the math says 30).
• Your wrench is reading low by roughly 10%.
• The Fix: In the future, if a manual asks for 100 ft-lbs, set your wrench to 110 ft-lbs to compensate.
• Write “+10%” on a piece of masking tape and stick it to the handle.

Option B: The “Calibration Nut” (The Risky Way)
There is usually a calibration nut hidden under the end-cap or sticker of the handle.
Tightening this nut compresses the spring (making it read higher), and loosening it does the opposite.

• Warning: This is sensitive. A 1/8th turn can change the value by 5-10 ft-lbs.
• Unless you are patient and willing to re-test 10 times, sticking with the “Correction Factor” method is safer.

Setup Testbed