Bosch e-bike speed wrong

I’ve just got a new (secondhand) cargo bike with a CX motor, from around 2019. All seems good, but having checked the speed with a GPS, I’m pretty sure it’s reading about 5km/h faster than it should be, and so cuts out sooner than it ought to. I’ve tried adjusting the wheel circumference in the Intuvia display, but it’s still reading fast as there’s only limited adjustment available.
I don’t really want to fit a speed sensor chip as I don’t want to make it illegal, they seem to mess with the speed display, and also cost a lot. Has anyone managed to fix this sort of issue in a legit way?

You can have it tweaked by a Bosch dealer, but only by a Bosch dealer. They’re truly scummy in the way everything is closed off even to other parts of the cycle trade.

Just accept you need to put more effort in to go faster so enjoy the scenery and take more in at the slightly reduced speed?

It’s a cargo bike. Take it easy.

They’re truly scummy in the way everything is closed off even to other parts of the cycle trade.

Depends if you want warranty in tact or not.

Seen some of the efforts at tightening stem bolts on here. Wouldn’t trust them with a 9v battery never mind an ebike battery

Mine used to cut out at a displayed 16.7mph and I’m fairly certain it was over reading so was probably  cutting out at the required 15.5 (cycled alongside someone with a Garmin to compare so not a conclusive test). It definitely logged different distances eg I did a sportive with 5 other people and we all covered exactly the same distance yet mine was significantly longer. Adjusting the circumference made absolutely zero difference, I don’t even see the point of that setting as it literally does FA.

it wasn’t the head unit either as I had a Smartphone Hub initially and then changed it to a Nyon. Both exactly the same.

but having checked the speed with a GPS,

I think that’s where you are going wrong. My Garmin has my speed wildly fluctuating for no apparent reason even when the view of the sky/satellites isn’t being blocked.

If you’re that concerned about it, check the speed by another method. Is there somewhere local which is flat and straight over a known distance (eg a kilometer or a mile) where you can actually time yourself? Or borrow a wheel magnet speed sensor type bike computer?

My brother’s ebike cut out slightly later than mine.
Pedalling a non powered e bike and trying to keep up with a powered one is a recipe for a heart attack!
Every fraction of an mph is vital.
How much does the reset cost.
I’ve got 27.5 plus wheels on my 29er and I’m missing 1mph.

in the settings in the app there is a speed limiter setting.  You cannot raise this beyond 15.5 mph can you can set it lower.  I wonder if someone has done this to your bike?