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I was having a think about how I can check the accuracy of my speedometer and I have come up with a suggestion, shoot me down in flames if you think it is a daft idea.

You guys who have GPS can easily check if the analogue speedo is accurate but I am still in the map and compass era.

So...

The way the system works is by a sensor on the rear wheel sending pulses to the computer, these pulses will be as accurately related to the speed as the designers calculations and the variations in rolling radius due to tyre wear and pressure will allow. So lets for the sake of argument say this is spot on.

Now the speedometer itself is an analogue device so somewhere there will be an D/A (Digital to analogue converter) this will output a voltage which then goes to the speedo. As we know from elsewhere, this can be changed by varying resistor values. So the accuracy of this unit is in question.

Now we also have an on board computer that displays average speed, this is a digital device being fed from the same wheel sensor.

So the process is, find a good long straight flat bit of road, get up to say 60MPH and engage the cruise control. leave it a few seconds to settle and then press the reset on the On Board Computer whilst Average Speed is selected. Again leave it settle for a few seconds and you should now have a fairly precise actual speed reading that you can compare to the speedo reading.

With my speedo sat at 50MPH my OBC stated 44.9MPH so roughly 10% fast.

At some time when I have the bike apart I will do the resistor mod to hopefully make the analogue speedo more accurate but until that time I will have a good idea of my actual speed.

May be of use to someone ...

Regards

Tony
 

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Very ingenious idea !
I do not think I would that thought of that and what a great concept.
I use my Garmin GPS and my speedo reads about 5 mph fast
according to GPS.
Actually I use my GPS to monitor my speed over the MC analog dial
because it is too diifcult to fine 55 or 65mph on it quickly.

Thanks for sharing this information
 

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mine consistently reads 10% higher then actual. according to road side indicators (your speed is) and my GPS
 

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+1 on what tobiwan said, mine's the same...about 10% higher on the dash speedo than on my iPhone speedo app.

I have RAM mount for my iPhone so I just use it as my speedo.
 

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Simpler way - For me at least

Have stopwatch or timing device in hand. Get on Interstate or highway, preferably flat, with mile markers. You can test this at any speed, not just at 60. Get on cruise control at the speed you want. As you pass a mile marker start the timer. Stop timer at end of 1 mile. You want to accurately note the seconds it took to do one mile.

We are looking for an accurate MPH reading here to check against your spedo. There are 60 seconds in a minute, 60 minutes in an hour. Multiply 60 x 60 to get 3600. Lets say you ran through that mile in 51 seconds. Divide 51 into 3600 and you get 70MPH. Check that against your spedo to see where you are. My spedo was about 9-10% fast in checking this.

I've done the spedo mod on my '99 LT by jumping the two terminals on the back of the board. My spedo is now within about 2 tenth's of a mile per hour against my Zumo. That's with Metzler 880's and them inflated to 42 front and 48 rear. I feel my Zumo is very accurate so I'm comfortable with my speedometer now.

Hope this helps
 

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I compare with GPS.
I set the cruise at whatever and then look at my at my GPS. Mine is off by 5-6 mph. I am used to it now and think if I don't mess with it to get it spot on it could work in your favor if pulled over by State man and the GPS has the lower reading. The FIX-IT is above my pay grade
ride safe :bmw:
 

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I also compare with the GPS. I wouldn't say my speedo reads constantly 10% faster, but it stays between some 5 to 8 km faster.
 

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hallzee said:
BEWARE the "Speedo Fix".... Click Here
Now that's funny. Obviously a speedo malfunction, and not due to the correct fix. The mod was how BMW made their Police bike speedo calibration correct. Good enough for me.

It looks scary, but next time someone is taking off the tupperware, look at the back of the speedo. All of the terminals are color coded for wire locations, and one lug even takes two leads. Take detailed pics, remove wires, remove the two nuts on the front outside corners, and send it to me. I make fix, and FEDEX back for $30...that includes shipping.
This is also a good time to upgrade dash lights to LED.
 
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