I swopped out the standard 16" front suspension with a complete Suzuki Bandit system. I have some idea of the speedometer variance after running an app on my phone called DigiHUD. Had to cabletie the speedo cable to the unit (Suzuki different to standard) but it's working fine. Seen some postings about GPS speedometers but don't really fancy adding all sorts of "stuff" to the instrument panel, bar a dual USB connector which was mounted just under the fusebox on my FJ1100. Out of interest I've recorded a 10km/h lower speedometer speed compared to the GPS reading at 140km/h. Would assume that this is standard when switching from 16" to 17" front?
I'd expect some variance, and 10% sounds about right. To me, a GPS speedometer would be an ideal fix. They come in sizes to exactly replace the OEM instrument, with a swinging needle or digital, or just add a cheap rectangle digital display to the panel. I use the cheap digital display on my bike because it is easy to read, and very accurate when riding slowly in towns.
Where the Suzuki cable attaches to the speedometer, you may be able to replace the Suzuki cable nut with the old Yamaha cable nut. Cut one side of the Yamaha cable nut with a hacksaw, then spread it open slightly and remove it from the old cable. Install the Yamaha cable nut on the Suzuki cable, and crush to fit. A cabletie can keep the Yamaha cable nut held to the correct size. If all that seems like a working idea, then cut off the Suzuki cable nut. Install the "new" Yamaha cable end to the speedometer in the normal way. HTH.
The oem FJ 16" front is 120/80-16.... Assuming your new tire to be a 120/70-17
There is not much difference between the two...less than 1%
Rev. / Mile: (16) 855.98 (17) 853.91 -2.07
Section Width: 4.72'' 4.72'' 0''
Sidewall Height: 3.78'' 3.31'' -0.47''
Tire Circum: (16) 74.02'' (17) 74.2'' 0.18''
Tire Height: (16) 23.56'' (17) 23.62'' 0.06''
If your speedometer reads 65 you are going about 65.2 mph
https://www.calculator.net/tire-size-calculator.html
If your speed readings are 10% off...then the discrepancy is not from the 16" vs. 17" tire differences but elsewhere.
MOST motorcycle speedometers - especially of that era - read about 10% faster than reality.
More recently, sport bikes have been known to have tachometers that read high ...
In the UK, the law states that vehicle speedometers must not under-read at all, and should have an error of no more than +10% over-reading. On my FJ with a 17" FZR front wheel and a new Michelin 120/70x17 radial tyre, at 36psi pressure, compared to my Garmin GPS Speedo, my FJ reads 2-3mph high at 70mph and around 5-6mph high at 140mph.