Since there are no "easy adjustments, the two ways to easily deal with it is to either get a front tire that has a larger overall diameter (they're not all the same, even if they have the same size designation) such as a Metzler ME880 (great tire btw). Or you can source a Thruxton speedometer drive from a MC salvage yard such as Pinwall. They are set up for an 18" wheel and that will correct things by turning less rpms per mile...
I run ME880 fronts, stock size and they give me just about dead on readings when I go past those "you speed is" radar stations...
UK law (and probably elsewhere) allows for a 10% speedo error (reading high) to allow for different tyre sizes and wear and bragging rights or use a sat nav.
You can't even really trust the nav systems either. My Garmin reads up to 4 mph off sometimes. It also varies depending on if you're on flat land or in the hills. Long story short, are you sure the readings are off?
I thought that my '06 was off 4-5 mph, but after doing my own calculations, I realized with my stock tire, at 60 mph it was only 1-2 mph wrong.
I knew it was not correct just based on what traffic around me was doing. I'd be doing 15 over in a 45 zone and still getting passed
So I used a simple speedo app for Android on a long straight stretch and held a steady speed.
The speed given by GPS is pretty accurate if held steady for say ten seconds. It has to smooth your velocity a bit as it calculates it. It could be off by couple percent, but not ten.
I'm used to the speed being off, but I hate that the odometer has the same error. And all my mpg calculations have a fudge factor.
They all read fast on those bikes. It's a nice safety margin when speeding, sometimes keeping you just under the magical speed that will cause the officer to pull a u-turn after you.
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