Yours is an old bike with what I imagine will be the old style either totally mechanical (like clockwork), or it may have a drive cable which gets converted into electronic pulses. I've re-calibrated the last type before in cars I've owned.
There are three types of error problems. The first is an 'offset error' which is when the reading is always the same in widths of the needle at each point around the scale. That is the easiest to get right ( move pointer or adjust hairspring). The second type of error is a percentage non-linear type where the speedo may read correct for the low speeds but then runs rapidly out at higher speeds. The third type is non-linear error where the speedo reads correct over 1/3 of the scale, something different over the next third and another error over the last third - that is the most difficult error to correct.
Yes, tires and the rolling circumference make a difference, but this would be a 'linear error' more easily corrected.
The only way to check and adjust a speedo is on a test bench. If it is completely mechanical it should have good accuracy unless the mechanism is fouled up or dirty. For years the leos here used large 'Chrono' speedos they claimed were linear, giving 1% accuracy and were like clocks inside. Of course on the vehicle that 1% would change with tires and offered a good defense! A lot of the old pre 60's 'Smiths' speedos were made like this. Mechanical speedos are checked with an adjustable speed test motor and accurate rpm meter, electronic speedos are tested with pulse generator equipment.
Then electronics came in to get them cheaper and speedos became part mechanical and part electronic. With some electronic speedos you can attach a pulse generator inside to check each of the marks around the scale. The dial is usually a large angle moving coil meter. That's where the problem starts because the voltage applied may be linear and correct but the meter doesn't move to the right place. So they usually fit a thin circular metal cup around the meter magnet which has finger segments. By bending the segments you can change the magnet field at a dozen points around the scale - that is how they deal with non-linearity. Since these speedo meters are the common moving coil type, there is a hair spring at either end of the moving pivot. Just like mechanical watches, these can be adjusted to offset a zero shift and give the right tension.
Most vehicle speedos are optimistic which gives a 'feel good' factor in lower power cars and builds in a safety marging for speed limits. Perhaps I'm biased, but most cars with engines under 1600 cc seem to have optimistic speedos.
If you want to avoid cluttering up the bike with addons that either have no backlight or aren't that weatherproof, I'd look at the speedo re-calibration option first. The alternative is to add some calibration marks to the speedo face for important speed limits in your local.
It's a brave rider that would ride through radar speed checks several times to see if the calibration was correct!
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