tyre size

[matt_400bandit]

would a 120/60 front tyre make my speedo severly out or just a couple of mph can anyone work this out for me because i dont no how also what effect would this have at 100mph ?????? cheers

[yuckhil]

http://mc.bridgestone.co.jp/en/products/battlax/bt92.html

Bridgestone lists overall diameter of 110/70R17 and 120/60R17 as 592 mm and 576 mm respectively, for the BT-92 model.
The 120/60 tyre is 2.7% smaller than the 110/70 tyre.
The circumference (pi * d = 1809.6 mm) is also 2.7% smaller
The 120/60 tyre completes a rotation in 2.7% less distance (1.8096 m).
At 100 mph (= 160.93 kph = 2682.24 m/min), the 120/60 tyre rotates @ 1482.26 RPM, which is 2.7% faster.
This causes 2.7% more turns /unit time through the speedo cable, which should increase the speedo reading by 2.7%, to ~103 mph.

Yay for high school math!

[gsxr400 racer]

my head hurts now

[dhcolesj]

But the front tire has no affect on the speedo, does it?  I didn't think the front tire was were the speed came from.  Maybe I have it wrong for the B400's.

I just thought I'd help gsxr400 out a little.  hehe.

[Red01]

I'm pretty sure the B4 is like the 1G B6 & B12 and has a cable driven speedo - that read front wheel speed. 

[PitterB4]

I'm pretty sure the B4 is like the 1G B6 & B12 and has a cable driven speedo - that read front wheel speed. 

Yup!

[dhcolesj]

Hmm, well I stand (or sit while I'm typing) corrected.  Thanks!