Modern Vespa

I have a 2023 GTS 300 HPE 2 Super with around 8,000 miles on it. And recently the top speed has been reduced by 20 miles from 98 to 78 (at least that's what the speedometer claims). I noticed the reduction of speed began right after the 6,000 mile service which occurred at 6,600 miles. Could this be a transmission issue? Perhaps the belt was not installer correctly? All of the work has been done at a Vespa dealership however I have serious questions about the quality of work at all of the Authorized dealers even before this.

98mph on a GTS 300 is a stunning almost 160kph.
It normally does not go that fast.

The GTS has a speed limiter and 78mph or 126kph is a more realistic top speed.

Did it really go 98mph on the speedometer? Did you ever check that speed on a GPS?

I did the abs tone wheel replacement on my 2020 SuperTech and the fastest I've gone down hill is 82 and its sketchy, I dont think theres any way you went 98 on a stock Vespa lol

My speedometer on my 2023 is approximately 13 mph off, so I do not seriously believe I was doing 99 mph. I own a 2020 GTS 300 HPE Touring and that speedometer was only 4 mph off and it claimed I was doing 84 mph.

Usually, the speedometer error with motorcycles and scooters is a consistent percent figure, so that would mean that a bike with a 10% 'optimistic' error would read 5 mph higher than actual speed at an indicated 50 mph, and read 110 mph at an actual 100.

My '20 GTS 300 HPE has pretty close to a full 10% speedometer error. I think that my 300's top speed to be about an actual 75 mph on a level road with zero headwind, though it's speedo needle would be showing 80-83 mph.

My Triumph motorcycle's speedometer is off at about the same percentage that my Vespa's is.