What conspiricy theory?

I've seen a lot of talk about speedometer inaccuracy ranging from the believeable "they just lose accuracy afte 25+ years" to the crazy "it's a plot between bike manufacturers and the governement to trick you into slowing down".

I just GPS'd my bike today and my speedo was dead-nuts accurate all the way to 75 mph. My speedo tops out at 85 so I can't verify it beyond that, but I seriously doubt my accuracy whacks out in the last 10 mph before the maximum reading.

Once I replace my front tire, I plan on adding a post to the "How fast have you gone?" thread.

 

Mine was dead on too, all the way up to 85. Verified with a TomTom One GPS.

 

My Seca 900 indicates 115 MPH while my Garmin E-Trek says 104.
60 is really 55. I guess the spring got weaker? To read low, the magnets would need to get weak, right?

 

Nah, theres a spring inside most speedos that provides resistance against the drive of the speedo cable. Mileage goes up, along with age, and the spring can weaken. At least thats my understanding of how these speedos work.

 

I've never GPS'd mine, but my bike is dead on with the local radar signs. They just wear out over time. My father's 79 CB750 reads about 10-15 km/h faster than he's actually going.

 

Ace, and Time-

As usual you have provided an insight which I lacked. I recently installed a new gauge-head that I scored from eBay. It only had 9k on the clock. It stands to reason that with so few miles, the return spring on the needle hardly has any wear and would read fairly accurate.

Now if I'd been able to try this with my old cluster that has over 50k on it, I bet I'd get quite a different reading.

 

i didnt know there was any alleged conspiracy re: speedometers lol...

 

The theory is solid. The spring (D) is the resistance against which the cup (C) is being pulled by the spinning magnet (B.)

It stands to reason that if the spring gets "tired" it will read high, and read low (or bounce) if it's gummy.