Caveat: I've never seen the details of the speedo's functional design nor the inside of one. The following explanations of how the calibration can get out of whack and whether the problem will present as a constant offset or a change in slope of the relation between magnet rpm and indicated speed is based on simple physics and the descriptions and pic's shown in some of the notes below. Since I'm an engineer, I always look for root causes... what changes and what effect that change will have. I offer the following for general information on what causes what kinds of changes in the speedo's indicated vs actual speeed when a calibration changes.
The variables in the speedo's method of indicating the analogue speed is thru 2 primary forces.... an electromagnetic force and a spring force.... both are invariates.... i.e. neither should change over time nor amount of use... since both are material properties, not settings or adjustments that are made or which can be made outside of cutting new metal. Material properties are constants unless the material has undergone excessive environmental exposures i.e. become badly rusted (oxidized) in the case of non-stainless varieties of steels (iron containing metals).
The electromagnetic force is created by a solenoid effect... magnet spins, generating a non contact electromagnetic force on the steel (iron) disk.
The disk is attached to a helical spring or other shaped spring, since otherwise it would spin continuously limited only by it's frictional components.
The spring force acts against the magnetic force applied by the magnet to the disk. As the rpm of the magnet increases so the magnetic force increases proportionately.... therefore the force to spin the disk increases proportional to the rpm. The helical spring force acts against the magnetic spinning force, but at a lower force at it's neutral (0) position. The difference in the helical spring constant (force vs displacement), and the magnetic force of the magnet acting on the disk let's the disk (therefore pointer on the dial) move thru an arc which is proportional to the magnet's rpm.
There are several variables that can modify the forces.... mostly simply friction.... the disk's friction on it's bearings, the spindle shaft's (attached to pointer from disk) friction on it's cylindrical bearing surface, both of which increase the forces acting against the rotation of the disk... effectively adding to the spring constant of the helical spring. When this has occurred, to whatever extent, it changes the slope of the speed vs rpm relationship.... since frictional force will generally increase with increasing relative velocity of the contacting surfaces, and if for no other reason than the fact that any of the normal lubricant's used (oils, greases) increase viscosity with increasing shear force... i.e. relative speed of moving parts being lubricated, at near constant temperature.
The spring itself is a metal which will not relax it's modulus with time and the displacement of the helical spring when wound to it's fullest won't cause it to reach anywhere's near it's elastic limit, therefore the spring contsant over it's entire displacement range will not change with time.
Another source of a change in friction within speedo are at the bearings for the gears that drive the odometer... since those are also driven by the shaft's rotation, but other than create a constant drag on the shaft, this source of friction won't cause the speedo to have an offset or slope change with shaft rpm..... other than a momentary and temporary (less than a fraction of a second) lag .... & then only when initially accelerating from a full or near full stop.
The other variable that can effect calibration is the location or attachment of the helical spring to it's fixed end points... and if either of them move it changes the offset of the speed indicated.. i.e. the speed is always indicated to be Actual +/- x... x is constant at all speeds, since the spring constant hasn't changed... just the position of the position of the spring's zero force point.
The magnetic force on the disk can only change if the location (distance) of the magnet to the disk changes, and I don't know whether there are wearing members that will increase or decrease the distance over time and wear. However, if this occurs then the relation of the magnetic forces applied to the disk changes, so that the helical spring's countering force relation to the magnetic force changes, and this will cause a change in both the slope AND offset of the speed vs shaft rpm. Assumedly, if this occurs, the distance between the magnet and disk will increase, decreasing the magnetic force, thus making the spring constant of the helical spring relatively too large... reducing the indicated speed vs that actual speed at an increasing rate... i.e. the higher the speed the lower the indicated speed relative to the actual. If the distance decreases the converse ocurrs.
You can't change the spring constant of the spring without changing the spring's length, width, or thickness between the fixed ends of the spring.... shortening the length increases the spring constant, which adds to the countering force on the disk, and drops the slope of the indicated vs actual speed... lower indicated speed than actual with an increasing error as speed increases. Increasing the length of the helical spring between it's fixed ends does the reverse.
Generally speaking, I'd expect the frictional and shaft/bearing wear points to be the cause of most error in W113's speedo's. ...i.e. friction, therefore pure offsets, but not a change in slope. Unless the spring's fixed end points have changed so that it's changed the length of the spring, the spring constant won't change. ... so other than that I can't think of any means by which the slope of the relation would change unless the distance between magnet and disk have changed.
Could the magnet material itself lose some of it's magnetism over time? Possibly, if it's not a natural magnetetic material. If so, it would change the magnetic force acting on the disk, so that the spring's countering force would no longer have the same relationship as the magnitic force increased with increasing rpm....i.e. it would change the slope of the relationship (magnet rpm to indicated speed), as well as the offset between them.
There is another cause for change in indicated vs actual speed that has nothing to do with the speedo itself, rather the shaft connected to it. The shaft runs in a tube...and thus has frictional forces while turning in the tube. Those frictional forces restrict the ability of the shaft to rotate in the tube and retard the shaft for a period of time... small sub-second periods... as the torsion in the shaft winds up under frictional forces in the tube, the speed shaft is retarded in it's rotation momentarily... and when it winds up enough, the force to unwind overcomes the friction in the tube, and it' unwinds ... speeding up it's rotation relative to what it should be, and indicating a faster speed than actual. What you really see on the speedo dial is a wobbly speed indicator... bouncing back and forth around the actual speed. Lubricating the shaft in the tube with graphite will solve that symptom.... though it indicates a wear problem has occurred within the tube, so the graphite solution may not do much for the symptom if wear is extensive (and / or wear debris is extensive)... it will reduce the wobble, but not eliminate it, or if it eliminates it, it may not do so for long... a year's worth of intermittant driving, perhaps.
A speedo calibration shop knows the design relationship beteen spin speed of the magnet and indicated speed on the speedo dial.... either because they have the spec's or because they've evaluated a lot of "in calibration" speedo's over time. They spin the magnet with known rpm, and mesure the relationship. If the relationship shows an offset exists (after cleaning and / or replacing bearing surfaces), there has to be a feature in the speedo that let's them change the location of one of the fixed end-points on the spring attached to the disk... which either adds or subtracts the force beign applied by the spring to the disk... depending on how much offset was indicated.
If the slope's off, frankly, I'm not sure what a speedo shop can do to effect calibration unless there's an adjustment feature in the design that let's them increase or decrease the fixed length of the spring... effectively shorten or lengthen it.... maybe up to 5% - 10%?.
Normally, cleaning and/or replacing the worn or dirty bearing surfaces, with and adding a few drops of the correct weight lubricant should result in an almost immediate "recalibration" without changing anything else. Since I'm not familiar with design details, I can't describe what other adjustments there may be designed into the device for initial setting and calibration.
Longtooth
67 250SL US #113-043-10-002163
'02 SL500 Sport
