i was talking to one of my biker buddies, and he claims that the bike manufactures offset the speedometer by 7 % ( i guess for safety) is this fact or fiction. ex. if the speedometer says your going 100 mph your really only doing 93 mph
thanks hope i was clear

 

Fact. There are devices available (Speedohealer/YellowBox) to correct the error. It actually ranges from 5.5% to 8% on new bikes.

Sprocket changes, of course, make it much worse.

 

It"s not exactly any one percent for anyone maker... but he"s correct, the speedometer is wrong.

The more complicated answer? The manufacture can"t know what combination of events will be in place.
Will the tire be run at the proper preasure? How worn is the tire? Did the rider change the sprockets?
how much weight is on the bike?

First - to explain the speedometer. The bike counts how many times the rear wheel turns. They do this by counting
how many times the front sprocket turns. They know the stock front sprocket size, and the stock rear sprocket size.
They know the stock tire diameter, so they can do math and get the circumference of the tire. So - 1 spin = x feet. Sounds simple?

Well - if you are runnning low preasure on a worn tire, 2 up with heavy people, and your camping gear - the circumference
of the tire will be D IFFERENT than a skinny guy on an over inflated new tire. So - 1 spin of the front sprocket takes
you a different distance. Speed is distance over time. Change the distance you change your speed.

So they guess. Not only do they "guess", they facter in a safety margin. What"s worse? Accidentally going slow,
or accidentally going fast? They say slow. So - yes - they do systematically have over reading
speedometers - 100 km/h is not really 100km/h it is nearer 92-95km/h.

The n_ew problem? On car leases Nissan and friends are in trouble. Every mile over your lease
you pay for. What if you now know the odometer is now wrong for the same reason?
You"d have paid hundreds of dollars for miles you didn"t drive.

Back to bikes - if you c hange the sprockets then you c hange the meaning of one
rotation of the front sprocket... meaning you"ll throw the speedometer even further off...

Please note that all changes are a percent and not a fixed value - 5% e rror, not 5mph error.
This is significant, because 50 plus 5 is 55. 50 plus 5% is 51... The faster you go the bigger the error

Anyhow... the speedometer is wrong. You can buy a product (example speedohealer) to correct it.
On a stock bike forget about it. On a bike with modified rear wheel size or sprockets - you pretty much need it.

Cheers,
woot.

 

(found a way to beat the sql bug - replace all spaces with _) :-/

 

Almost all Odometers are correct from the factory. When you add a speedohealer, it corrects the speed but makes the odomoter wrong. Catch-22.

Check out the Speedohealer FAQs.

 

Mine is so far off now with the 520 conversion that I don't even bother to look at it. I might take the nav system out and see just how far off I am one day. When I go by those things the cops put out to show your speed I try to look but I never have time to figure it out.

 

On my last ride I brought my Garmin eTrex and speedo was correct up to about 45mph, then it started creeping.

At 50 on GPS, 52 on speedo
60 on GPS, 63 on speedo
70 on GPS, 75 on speedo

and so on...135 on GPS, mid-140s on speedo (I only glanced briefly, my attention was elsewhere!)

 

so if a stock f4i with a 520 conversion what kind of difference can i expect in change of speed.
example if im going 60mph

 

Depends on what you're geared. Mine with -1/+1 is around 14%.

 

dfgsdfgsd