Tire size and speedo Accuracy?

[GSXfs600]

I'm trying to determine if my speedo is way off or closer?

With the stock tire size(110/80/17) my speedo seems to read high?
It may read 80 mph but folks are passing me.

With the new tire(120/70/17) the speedo seems closer?
When it reads 75 mph I'm with the flow or just under.

Hes anyone tested this out? We dont have mile markers around here.


Thanks.

[Zepp]

Most speedos are about 8% high on bikes

[GSXfs600]

Most speedos are about 8% high on bikes

Why? So they can lie about mpg?

[thetable]

Speedo, not odometers are roughly 8% optimistic.

Not sure exactly why, but my best guess to to account for the fairly wide spread of actual tire diameters from manufacturer to manufacturer. Better to be on the safe side and have the speedo read a little fast. On cars where the specs are a little tighter, the speedo's are still usually at least 3% optimistic. When I had a newish car, I could pull data off the OBDII port that would match the GPS data almost exactly, but the speedo was off by ~4% on the high side.

[GregS]

Have you tried having a friend with a car, and accurate speedo, run at predetermined speeds and you pace yourself with your bike so you can get an idea of accuracy? You can sometimes even get a friendly cop to do this for you, tell him / her that you just want to be a good citizen and not go speeding on public roads.

[Psycho1]

I have about 600 miles on my new 120/70/17 Road attack front tire, and yesterday I tested my speedometer against my friends R1 speedo that was calibrated to within .5mph by gps with a speedo healer. When I was reading 60mph, he was showing 57mph. At 140 on my speedo, he read 133.

[Jax]

Ok so can someone explain how the speedo healer works on a post98 (computerized vs speedo cable on the front tire)? I just wanted to know how the healer actually worked and do you program it?

[Psycho1]

I wondered the same thing about the speedo healers, so checked out the one on my friends R1. It has a small dial that you turn that adjusts the signal going to the speedo to move the reading up or down. He mounted his to the handlebars for tuning purposes, along with his GPS, then turned the dial until they matched. Now his agrees with his GPS all the way up to top speed (170mph+).

[GSXfs600]

I have about 600 miles on my new 120/70/17 Road attack front tire, and yesterday I tested my speedometer against my friends R1 speedo that was calibrated to within .5mph by gps with a speedo healer. When I was reading 60mph, he was showing 57mph. At 140 on my speedo, he read 133.

So the tire change does get it closer! I can live with 3 mph off.


Heck! seating angle is a +/- 1 mph.

[Psycho1]

Yeah mine was a lot closer than I expected it to be. That means with the stock gearing when the speedo hit 150, I should have been going around 142 still. Not bad for a 600 Kat. Don't forget different brand or model tires could have different overall circumfrences even if they have the same size listed on the sidewalls though. One 120/70/17 might read 3mph off at 60mph indicated, while another brand might read 7mph off.