Most factory stock bikes, overstate mph by 5 to 10%..... ours included...(They get a double advantage with this... owners think their bikes are faster,... and they can't be sued over a speeding ticket)...
...(ODO's,.. although they are powered by the same cable, work differently and are much more accurate)...(ODO's are a direct mechanical connection.. speedo's have some sort of excited coil doohicky. I could explain further, but what's the point, when neither of us would understand?)...
...
...but,.. it's not just the stated tire size...
As a tire wears, it's circumference changes and speeds read different...the amount of air also varies... and the centrifugal force as speed increases, makes tires taller...
(this is why tires can seem fine when installed, but rub at speed)...
So, basically... no speedo reads accurate...
...(even scarier than that,...no police radar does either)...
... (they need to be re calibrated at regular intervals)...
(I don't even trust GPS to be truly dead on 100%)...
The only sure measure... is a measured distance and a stopwatch...(and that requires a constant speed)...
Anyway... ...yes. anyway...
A 140/90-15 gives a closer to accurate mph than stock... (you ain't distorting it, yer' correcting it)...as our stock reading, under...
...but,.. no two speed calculators will read exactly the same,.. ever...
Perfection is an illusion...
So don't worry about it... or if you're really worried, just ride under the limit...
I hardly ever look at at my speedo... (if I'm rockin', I know I'm speedin'... if I'm puttin', I know I'm not)...