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Discussion Starter #1 (Edited)
It is a well-known phenomenon that most motorcycle speedometers -- heck, most speedometers, period -- are not dead-on accurate. Usually, this means that your true speed will be somewhat slower than your indicated speed. The size of the discrepancy will vary depending on multiple factors, but if you're hopping on a bike unknown to you the odds are that the speedo will register something faster than you are actually moving.

To me, this is no big deal. For one thing, it constitutes sort of an insurance policy against speeding tickets. But otherwise, my speed is generally more regulated by road and traffic conditions than by the gauge. All the same, I like knowing if my speedometer has a discrepancy, and how big it is, and about the only way I know to do that is by using a GPS as a second speedometer for comparison, on the flat-out assumption that the GPS system is overall more accurate than your average speedometer. That being the case, I have speed-checked all three of my bikes with the following results:

-- 2019 Kawasaki Z650 -- Not bad. Generally indicates about 2 MPH faster than GPS-measured speed.
-- 2008 Suzuki Boulevard C50 -- Amazingly consistent. At any speed above about 30 it indicates precisely 5 MPH faster than the GPS. This holds true as far as I have tested it, which is up to an indicated 90 MPH (85 in real life), which is coming seriously close to as fast as it will go.
-- 2018 Dongfang DF250RTG -- The ringer in the bunch. No other way to describe this bike but cheap and Chinese (and also great fun). The speedometer is as simple as they come, and built as cheaply as one could possibly be made and still be called a measuring instrument. It's marked in kilometers per hour, but converting that to MPH is simple and reveals that it is absolutely accurate compared to many, many measured runs with the GPS.

I have no idea what, if anything, this finding signifies -- except that maybe being accurate is a fault in a speedo, and therefore to be expected in the cheapest one imaginable. :)
 

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2006 Honda CBR1000RR, 2008 Honda CRF230L, 2019 Honda CRF1000L
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My 19 AT reads 29, I'm actually doing 30. From at least 55-75 it's dead on with my GPS. Most accurate speedo on a bike I've ever had. I'm glad I checked it as I'm used to them being off by ~5mph at 65. My 06 CBR 1000rr certainly is. When it reads 70-71, I'm actually doing 65. It gets off farther, the faster I go.

My CRF 230l isn't too bad. I used to think I could do 72mph on it, on level ground. Put the GPS on it and that 72 showed 69 on my GPS.

If the gearing and speedometer are calibrated for a particular tire diameter, and you go larger than that diameter, your actual speed will increase vs what is being displayed.

My Yukon has stock diameter tires, the speedometer is dead on at any given speed. My truck went from a 245 to a 265 tire(taller), and it went from accurate to reading 65, but my actual is 66.
 

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All my bikes read around 2 to 3 mph over. The XS400 is quite accurate, which does not help me seeing 95. The SV1000 is the only one to have been on the dyno. The tach also reads high by about 500 revs. But at 155 mph and 11,000 revs, who cares if accuracy is 150 mph and 10,500 revs. Sharp readers will notice the dyno shows rear wheel speed. The computer is giving the numbers. The road speed was checked by GPS like the OP. GPS speed is quite accurate, height above sea level is not so accurate. On the boat we can compare GPS speed against know meter speed, and distance traveled divided by time, all with the variance of tides and currents. Sometimes a more accurate account of beers per mile works best. UK
 

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The speedo on my Indian was off until I put a slightly larger tire on. Next tire went back to the OEM and it’s off again. No biggy because I got used to watching my GPS instead anyway.
 

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The speedometer on my R1150RT is weird. Some days it reads 5 mph high pretty much across the range. Other days it reads exactly the same as my GPS. Some days I get both behaviors on the same trip. I haven't been able to relate it to weather conditions.

Here's something I found interesting: in 3rd gear my speed in mph (according to the GPS) is engine RPM / 100, exactly. So if I want to do all my riding in 3rd gear I can have a super accurate speed indicator without firing up the GPS.
 

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I think this is more consistent with motorcycles than cars but it is frustrating. I usually estimate most are about 5mph slower than the speedo shows.
 

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My C14 was about 10% fast... if it showed 33, you were going 30. At 65-66 indicated, you were going 60. At 100 indicated, the actual speed was about 90.

My new Versys 1000 has a speedo that's nearly right on the money.... within 1 mph at 70.
 

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My old BMW always read about 5 mph too much.
But my present 99 Goldwing seems to be spot on when
I check it against the GPS.
 

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Here's something I found interesting: in 3rd gear my speed in mph (according to the GPS) is engine RPM / 100, exactly. So if I want to do all my riding in 3rd gear I can have a super accurate speed indicator without firing up the GPS.
Now that has me curious about mine. So let me get this straight, in 3rd gear if your rpm is 4,500 you are doing 45mph, correct? I've never ever paid attention to the lower gears except the shift point rpm or honestly the sound. I'm not sure I actually know the rpm I shift. But now I'm going to have to pay attention just to see how close mine is to what you get. And why 3rd gear? Mid point gear maybe where it's close to ½ of 1 to 1 gear ratio. 6th gear on mine is not a 1 to 1 though, so not sure what this all means.
 

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2006 Honda CBR1000RR, 2008 Honda CRF230L, 2019 Honda CRF1000L
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Certainly not sportbike track gear's fault, they're sending a return label and exchanging this speedohealer. What pisses me off is I'd just got back from lunch, FedEx walks in, BS's a bit and lays the package on the counter. I'm sure he did that as opposed to handing it directly to me and getting an ear full.

62886

62887


Looks like it got run over
 

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^^^ Ouch! Looks like someone's boot heel did the "Twist and Shout" on that!

My biggest issue isn't necessarily that the speedo is off, although when toeing the line between speeding and reckless, I'd prefer to know FOR SURE! It's that the mileage is higher than actual as well. Throws off maintenance, sometimes by thousands of miles, as well as lowers the value a bit when reselling.

I installed a SpeedoHealer last summer and got it set a bit in the other direction, i.e. reads lower than I'm actually going, to offset the previous 20,000 (or is it only 17,000?) miles of "over" estimating...
 

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It is a well-known phenomenon that most motorcycle speedometers -- heck, most speedometers, period -- are not dead-on accurate. Usually, this means that your true speed will be somewhat slower than your indicated speed. The size of the discrepancy will vary depending on multiple factors, but if you're hopping on a bike unknown to you the odds are that the speedo will register something faster than you are actually moving.

To me, this is no big deal. For one thing, it constitutes sort of an insurance policy against speeding tickets. But otherwise, my speed is generally more regulated by road and traffic conditions than by the gauge. All the same, I like knowing if my speedometer has a discrepancy, and how big it is, and about the only way I know to do that is by using a GPS as a second speedometer for comparison, on the flat-out assumption that the GPS system is overall more accurate than your average speedometer. That being the case, I have speed-checked all three of my bikes with the following results:

-- 2019 Kawasaki Z650 -- Not bad. Generally indicates about 2 MPH faster than GPS-measured speed.
-- 2008 Suzuki Boulevard C50 -- Amazingly consistent. At any speed above about 30 it indicates precisely 5 MPH faster than the GPS. This holds true as far as I have tested it, which is up to an indicated 90 MPH (85 in real life), which is coming seriously close to as fast as it will go.
-- 2018 Dongfang DF250RTG -- The ringer in the bunch. No other way to describe this bike but cheap and Chinese (and also great fun). The speedometer is as simple as they come, and built as cheaply as one could possibly be made and still be called a measuring instrument. It's marked in kilometers per hour, but converting that to MPH is simple and reveals that it is absolutely accurate compared to many, many measured runs with the GPS.

I have no idea what, if anything, this finding signifies -- except that maybe being accurate is a fault in a speedo, and therefore to be expected in the cheapest one imaginable. :)

Since I started riding in 1970 bikes have about a 10% built-in error. At 30 mph, you're really doing about 27 mph. At 60 you're doing about 55 mph. They built in the illusion of a faster speed to help riders NOT get speeding tickets. Since gear ratios in the speedo can be made to be precise (mathematically) there is no reason they can't make a spot on accurate speedo.

For the ones that use an electronic speed sensor, same thing applies.
 

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2006 Honda CBR1000RR, 2008 Honda CRF230L, 2019 Honda CRF1000L
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I doubt there's any conspiracy here. Tires of equal size across various brands have different heights, it'd be impossible to account for that across the board.

My AT doesn't follow the 'built in' error. My crf230l doesn't come close to 10% off, not even half that.

I'm thinking it's mass produced and keeping costs down just getting it close.
 
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