Tyre Size vs Speed indication

[Highlander]

Evening all,
It might sound like a silly question but how much difference would tyre profile make to speedo indication? (speedo is driven off the front wheel)

We just got Mrs Highlander a late 80's GSX600F and the speedo is a bit generous. Indicating 120 Km/h when I'm follwing at 105, and my GPS indicates 100. Also following her around the road my trip meter and GPS are pretty close but her trip meter is about 10Km high over a 200 Km ride.

The "book" lists the front tyre as 120/80/17 (I think) but the tyre on it is 120/70/17
Could this account for the error?

[hayd3n]

very probable

[erik]

I don't think so. It should only make about a 4-5km/h difference according to some rough calculations I did. This also matches with my experience with my sv650s going from a 60 profile front to a 70, it makes about a 5km/h difference at 100km/h.

[Gremlin]

barring erik's geeky calcs, yes it will make a difference.

2nd number in tyre stats is the percentage height of width (first number). A taller tyre effectively has a larger diameter, so therefore, for a given speed would complete a rotation in more time.

Conversely, a smaller diameter would have an increased rotation speed, and you mention reading off the front wheel.

Now, how much of a difference... over to erik

[Jantar]

Most Suzukis over read by around 7 - 10%. That's why I fitted a speedo Healer. Unfortunately there is no inexpesive way to calibrate the speedo on M's bike. The lower profile tyre will add to the inaccuracy, but on the plus side, just don't tell her how far out it really is and she won't get any speeding tickets.

BTW how are you handling the HWB outage next week?

[klyong82]

I just chuck those tyre sizes on speedohealer online calculator and the difference is out by 3.8% and if you add the factory speedo error then it is out by 8.9%