It is certainly possible to adjust the speedometer, but then the odometer will receive false readings.
In Germany this would be an offense! (Too many dealers manipulated the odometer to cheat the customers.)
Well, to my experience in the automotive industr, today most speedo's are adjustable. It's due to the fact that varying tire sizes quite obviously affect the speed/distance a vehicle/travels travels with each tire rotation. For example, I could upgrade my tires so that I use the same rim, but have a wider diameter tire (taller side walls), in which case, with my current settings the speed would actually start to be a "bit more" accurate as I would be traveling further with each tire rotation.
For vehicles in the US you can actually purchase vehicle programmers that will calibrate your speedo to a custom tire size (if you didn't adjust your speedo, then if you put on larger tires, you'd think you were doing the speed limit, when in fact you could actually be doing up to 20%+ faster than the speed your guage reflects or smaller tires could mean you would be up to 20%+ slower), else possibly get a ticket for speeding or angry drivers for you driving too slow, dispite your speedo saying you're doing the speed limit.
Performance programmers from companies like diablosport.com, superchips.com, edgeperformance.com, hypertech.com, bullydog.com a few dozen other vendors all have the ability to change your speedometer calibration based on tire size. I personally own DiabloSports.com Predator II Programmer (for a 2001 F250 V10), SuperChips FlashPaq programmer (For a 2007 Dodge Ram 5.7 Hemi), Hypertech.com HyperTech III (2002 Chevy Silverado 5.2 V8) programmer, EdgePerformance.com Edge Performance monitor (2007 5.7 V8 Hemi Dodge Ram) on my past four vehicles. Trying to tune my vehicle to save on gas, yet get better performance as well (more HP). As well as program for my tire sizes for the speedo as I typically don't stick with stock rims or tires on my vehilcles.
I've got stock tires and rims/wheels now on the Kymco (always have had, and my odometer on the bike only has 1600 miles on it, obviously MORE than reality as the bike thinks I'm traveling faster over a greater distance than reality), so the error I'm getting is not due to any alteration I've done to the scooter. It seems the manufacturer (Kymco) chose a speedo setting that is made for a larger tire. OR, it's adjustable and needs to e correctly calibrated.