I put whiteout marks on the gears in question and rotated the output shaft (runs the front sprocket and on its other end it runs the little conical pinion gear that turns the speedo shaft) and counted 8 revolutions before the marks mated up perfectly again.

The pinion on the output shaft has 7 teeth, the driven gear on the speedo shaft has 8 teeth.

This is 0.125 shy of a 1:1 ratio or a 1 to 0.875 gear ratio   (output shaft pinion being smaller than the speedo gear, meaning speedo shaft actually turns a little slower than the front sprocket shaft turns)

Now that you know that the gearing thing doesn't work out quite the way you had planned, simply ask the accessory handlebar speedo guys which ratio they bought and got to work "correctly" for them.

               What makes you think these handlebar speedos are ever right?

And for further knowledge, those of us who have played with setting up digital speedos to the averaged distance covered off of many many consectutive interstate mile markers (averaging out all the distance errors to get a good true rolling land distance) will tell you a Suzuki stock speedo set up isn't perfectly accurate either.

On top of that, we all use different size rear tires (generally bigger) which further stirs the soup.