Stone the crows!
I think our mate from down under was pulling your leg, marble townsman.
The speedometer is driven from a wheel, so the sprocket and the position of the gearshift have no effect on the speed displayed. Understand that our stock speedometers are none too accurate at highway speeds.
Perhaps a 16-tooth front sprocket will make you ride faster. It all depends on your personal style. I went up from 15 teeth to 16 teeth on my Wee-Strom, and I think I burn less fuel than I did. For me, that is good, because I contemplate some rides where I go 380 kilometers between pumps!
There are several ways to deal with the stock speedometer's inaccuracies. I use my right wrist.