Speedo Error Margin w 17 inch front?

[micklm]

Hi there,

With regard to the speedo calibration issue when using 17inch tyres.....

If you use a 120/60 tyre on a 17inch front wheel then the difference as compared to the OEM 130/60 on a 16inch front is only about 2.5% (linear deviation, comparing circumference). ie. 287.9mm vs 281.2mm circumference

And I think it is +2.5% vs the typcial -5% to 10% conservative speedo error you get from the factory at highway cruising speeds (100km/h ish).

If you use a 120/70 front tyre like the 907 the difference is larger - about 6.5%.

Has anybody tried a 120/60 front on a 17 inch wheel and then GPS checked the actual speed vs the OEM speedo. It might be pretty close - better than factory even?

You would have to make an adaptor to match the OEM Paso speedo to the 17inch wheel hub, but it can be done (I have prototyped one for a 1994 SS wheel and it works).

Anyway, my numbers are usually pretty suspect - have I made a stupid mistake somewhere?

Thanks,

Mick

[paso750]

Mick,

a 120/60-17 shows -2% on the speedometer compared to a 130/60-16. A 120/70-17 -6%.
Speedometers are usually in the 0 to +7% range.

G.

[micklm]

G,

Thanks mate - much appreciated.

I'm going to go with a 120/60-17 front tyre and GPS check the accuracy with the OEM Paso speedo through the speed range to get any error changes mapped out. I've noticed on most bikes the error is non linear with speed. Definately more conservative at 100 - 120km/h than at 50km/h.

Will post results.

Cheers,

Mick

[ducinthebay]

two things.
1. I ran a 120/60 and hated it. Stood up in the corners, and didn't turn in well. Switched to a 120-70 (same brand and model tire, Dunlop 208) and everything worked out lovely.

2. When I switched to 17 wheels, I used the speedo drive from the 17" wheels. I had to machine the inside of the left leg to get the wheel centered, same as what was done on the 907. I have checked it at 45 mph from a roadside radar sign and it was right on. Since the Sport used the same wheels, and I assume the same speedo drive, I would assume the same would be the true for the Paso. My ST4s is doing 65mph when the speedo says 70mph, so the old Sport is better than the new bike.

[micklm]

Hmmmm,

Food for thought. Now that you've said that I remember I used to run a 120/60 on my 600SS and then switched to a 120/70 and things improved. I thought it was the slight adjustment in trail (i dropped the forks a bit at the same time) but I wonder now how much a role the aspect ratio of the tyre was at play.

I'll try it and see I suppose I have a 120/60 tyre waiting so I'm past the point of no return - for the next few thousands of kms anyway!

Cheers for the input,

Mick