Modern Vespa

So as my speed goes up the error become by up to 9 km/h (I am an expat in Korea) it starts at around 4 then at around where the speedo indicates 100 km/h it is actually 91.

I know there are discs that can be replaced to make this more accurate with the manual gauge on the Sei Giorni II, but is this a warranty issue?

No.
Most speedo are 5-10% optimistic on 2 wheels.

Edit: And welcome aboard. Post a few pics of your Sprint and some of your local scenery. We love some photos.

Due to legal requirements, speedometers in Europe must always display more than the actual speed driven (e.g. lt GPS). Therefore, this is not a fault and will not be fixable with original parts.

As written, in Europe a speedometer advance is mandatory by law, applies to motorcycles, cars, trucks, buses, etc. (Everything with a road registration).

Thank you for these responses. I guess it is better to indicate faster speed than getting ticketed all the time. I think I will install a GPS based speedometer sometime in the near future.

Piaggio speedometers all seem to read about 10 percent optimistic. It's easy enough to mentally deduct 10% from the speedometer reading to estimate the true speed within a couple of mph (or km/h).

Somebody on AF1's Scarabeo message board put together a re-scaled speedometer disc that you could laser print on a piece of vinyl, cut out, and stick on the speedometer glass.

With a little bit of trial and error, it works perfectly. Mine has lasted 18 months so far with just a bit of fading.

The search isn't working right now or I'd post the link.

I have a bicycle speedometer that instead of using wheel mounted magnets to measure speed uses GPS.

It has no wires. It is self calibrating and exactly accurate.

USB rechargeable.

I mounted it to the headset with Velcro.

Yeah I have been looking into GPS based speedometers lately. Will probably get a simple bicycle one as I usually ride around the city and really need this occasionally on longer weekend trips.

Also helps to make all figures look better for the manufacturer.

Mpg seems like you go much further for every litre of fuel.

Servicing is performed at a quicker interval than stated.

Vehicles odometers rise quicker, meaning secondhand vehicles are less desirable in comparison to new ones etc.

0 to 60 seems quicker. Top speed seems higher.

You can use your phone with Waze, like this:

my measurements on my supertech show odometer and trip etc are correct compared to gps - it's only the speedo that lies

Yes and no…

I'm talking about speedometer not necessarily about odometer. Depends on used method to measure speed/distance while driving, meaning a speedometer advance will not necessarily run the odometer "faster". However, I can not exclude the possibility that there are vehicles where speedometer and also the odometer runs "faster".

But: Fuel Consumption & emission values per kilometer (which have an influence on the tax in Germany), top speed and acceleration for advertising/brochures/website/etc. are measured according to a standardized and calibrated procedure, and it is completely irrelevant what the speedometer and odometer of the vehicle indicate.

In German vehicle documents (and I think also Europe-wide) is the maximum speed measured according to the procedure described above. For the Vespa GTS 300 (pre-Facelift = 120km/h; MY23 Facelift = 123km/h); this coincides 1:1 with the GPS measured value (for the pre-Facelift exactly until the speed limiter kicks in).

But you can easily check the odometer reading: Plan a route, set the trip odometer to zero/memorize the odometer reading and drive the planned route, I'm pretty sure that will fit as far as possible.

Wrong. The odometer is surprisingly accurate - it's the speedo that's off. Whether driven by old-school induction or more modern stepper motor, they have to by law never show less than the actual speed, and must never show more than 110% of actual speed + 6.25mph (UK) or 110% of actual + 6kph (EU).
Piaggio's speedos adhere to the EU regs.

can't help but wonder if that is staged

Same here on three separate GTS.

I recently purchased a small(?) GPS speedometer from Amazon for $15 (USD), thinking what the hell, if it works it's a cheap 'fix' for both my Vespa's very inaccurate speedo as well as my Triumph Rocket 3's equally fibbing speedo, as it can be used on both.

It works surprisingly well (considering that for $18, I didn't really expect it to work at all), although it reads two mph above what my quite trusted phone GPS app reads, and one to one and a half mph above what our 2016 Miata MX-5's speedometer reads, which, amazingly, in this day and age, reads one mph BELOW it's actual speed.

I guess it was worth buying, but it's still a couple mph off of actual speed, though that is still quite an improvement over my Vespa's speedo, which is something like 5-7 mph optimistic at 65-70 actual mph.

I'm not convinced that even at $15 if it was worth it. I generally just subtract 5 mph from whatever the Vespa and Triumph speedos read.

Likely wouldn't survive heavy braking.

Or the 4wd tracks that i love

FWIW I put the Kubler speed ring on 3 different GTS , 2018, 2019 and 2023. It made zero difference in top end for any of them unlike with the 2020-2022 HPEs but it did correct the speedo to between 1 & 2 mph from my Garmin Zumo GTS. Okay it cost me $40 instead of $18 and was installed when I did a tire change but I didn't have to stick anything in to the scoot.