Speedo drive is off the trans since mid 1990s. Perhaps only the springer softail still maintained the hub drive and a cable drive speedo. All else is electronic. Front tire size will not effect the speedo readings.
Changing the rim size to a larger diameter but maintaining the stock tire height gives the illusion of bigger tire. It uses a smaller size wall which can take away from ride cushioning but reduce sidewall flex and improve cornering and handling.
It can be expensive. Most rims aren't OEM and until recently, you only had the choice 16, 19 or 21 inches. Tire sizes were limited too and until Harley started to use 17s it wasn't until now more options opened up as 17s are popular with the sport bike crowds.
18s make a great substitute for 16s on touring bikes. I had the chance to get 18 inch spoked wheels for my classics and regretfully passed as the price was $500 for the pair......used...with rotors. Some days.... I love this economy.
“When the going gets weird, the weird turn professional....” ~ Hunter S. Thompson~
R.I.P. Rubberdown. 1971-2012