Speedometer Off By 5%

[W14]

Installed a GPS yesterday and was curious as to the accuracy of the C-14's speedometer. What I found was that speedometer was high by about 5%. At 40 mph the GPS read 38; at 80 mph the GPS read 77; don't know about speed higher than that.

[Pokey]

Yes that is correct, and pretty much the norm for any bike I have owned.

[W14]

For Harley's as well?

[lt1]

Manufacturers are capable of creating accurate speedometers.  It's more of a legal/regulatory issue.  There is a lot of leeway for optimistic speedos, but none for pessimistic ones, so pretty much everybody plays it legally safe rather than accurate.

[Breaker19]

For Harley's as well?

Yes. The Ultra Classic I had was off about that much too.

[koval68]

190/55 rear tire will correct speedometer error quite nicely actually, just saying.

[Pokey]

190/55 rear tire will correct speedometer error quite nicely actually, just saying.

Really?

[DGOLD]

I thought the speedo was connected to the front wheel, if so how does changing the rear do anything to the speed?

[Jeremy Mitchell]

I thought the speedo was connected to the front wheel, if so how does changing the rear do anything to the speed?

The only sensor on the front wheels is on ABS bikes.  The C14 has the speed sensor around the primary drive IIRC.  I would have around a 5% error with the 190/50, with the 190/55 (about 1/2 inch taller) it is more like 2-3%.

[koval68]

The only sensor on the front wheels is on ABS bikes.  The C14 has the speed sensor around the primary drive IIRC.  I would have around a 5% error with the 190/50, with the 190/55 (about 1/2 inch taller) it is more like 2-3%.

My observations exactly!
Speed sensor is located on top of front gear case.

[jjsC6]

I thought the speedo was connected to the front wheel, if so how does changing the rear do anything to the speed?

That is exactly the point.  The speedometer reads the same as the front wheel continues at the same speed.  But a taller rear tire reduces your RPM's at the same speed, or conversely raises your speed in relation to RPM and front tire rotation.

Mine read the same error rate and at top speed it is about 9 mph high.  BTW, with the 55 tire mine is still off 1 mph at 70. 

[So Cal Joe]

I had a 190/55 on the rear of my 09 and it brought the speedo closer but not right on,

[lt1]

That is exactly the point.  The speedometer reads the same as the front wheel continues at the same speed.  But a taller rear tire reduces your RPM's at the same speed, or conversely raises your speed in relation to RPM and front tire rotation.

Mine read the same error rate and at top speed it is about 9 mph high.  BTW, with the 55 tire mine is still off 1 mph at 70.

I'm not sure if I follow what you are saying.  A taller (or shorter) rear tire will affect the relationship of rpm's and actual speed, regardless of the speedo drive pickup.
But ....
1 - The C14's speedo is not driven off the front wheel.
2 - On a bike with the speedo driven off the front wheel, changing the rear wheel size (or gear ratio/sprocket sizes) will have no effect on the displayed speed.
3 - On a bike with a rear-drive speedo, changing the tire or sprocket sizes will affect the displayed speed.

[The Pope]

If you want to correct your soeedometer, get a Speed-O-Healer.

http://www.healtech-electronics.com/

Fred H. did a write up on this : http://www.pbase.com/fredharmon/speedohealer

[Conrad]

If you want to correct your soeedometer, get a Speed-O-Healer.

http://www.healtech-electronics.com/

Fred H. did a write up on this : http://www.pbase.com/fredharmon/speedohealer

I never understood the logic behind this. Why spend the money and go through the trouble of installing one of these things? You know that your speedo is off a bit but so what? I know, it's a farkle...

[martin_14]

I never understood the logic behind this. Why spend the money and go through the trouble of installing one of these things? You know that your speedo is off a bit but so what? I know, it's a farkle...

  that's enough reason for most of us (me included) but as Fred put it: a gauge is there to give me information. If it doesn't do that, it's useless.

[maxtog]

I never understood the logic behind this. Why spend the money and go through the trouble of installing one of these things? You know that your speedo is off a bit but so what? I know, it's a farkle...

Some people just like to have an accurate speedometer.  I know I wish MINE were in all my vehicles.  I am not sure I would expend that much money or effort to correct it, though.

What would be nice is if we could calculate the exact error with GPS, and then have the dealer simply adjust the computer to make it correct.  No wiring, no installation, no box to fail, etc.

[Conrad]

I just go by what my speedo says. If in reality I'm not doing 70 but 67.2, or whatever, I'm fine. To each their own right? Besides, I'm usually speeding no matter what*.


*never in a school or construction zone though.   

[Pokey]

I honestly never give it much thought, I look at my GPS most the time anyways.

[The Pope]

On bikes with chain drive, if you swap out either of the sprockets, it changes the speedometer reading. That's where the Speed-O-Healer is needed the most. But then there's the guy that "must" have his sruff perfect!