Speedometer is 10 mph off

[Kingjoel]

First of all, thanks to the great, helpful people on this forum! My 1978 GS 1000's speedometer reads 10 mph faster than the bike is going. I know this from the GPS on my phone. I just learned the tire size should be a 3.50 -19 where I have a 3.25-19. My guess is that's the difference (albeit a big one). I will now try to find out how to donate to this forum, I love it!

 

[sharpy]

1st up a 3.25 x 19 IS right size (Metric size is 100/ 90 x 19) Thats still a 90 profile so it isnt a issue with your tyre. And who told you a 350 tyre is right size. that would make it steer like a truck.

 

[jsandidge]

Many people here (maybe all?) have said their speedos read 5-10 mph fast.

 

[Sam 78 GS750]

Not sure how accurate the GPS on your phone is, but speedos in pretty much all vehicles read fast by design. 10 MPH seems excessive though.

 

[Steve]

Whether you look for a 3.25 or a 3.50 doesn't matter, they are almost impossible to find in today's market. Look for a 100/90-19.

Most of the speedos hover in the range of about 5 mph off at 60 (about 11%), using a 100 tire removes most of that. If your speedo is off more than that, you will likely have to live with that.

We suspect that dpeedos were intentionally inaccurate from the factory. Joe would say "my bike (with an accurate speedo) goes 105 mph". Bob says "my bike (with an inaccurate speedo) goes 110 mph". Guess who has the "faster" bike? Guess who has bragging rights, even though the bike might be slower, due to the wrong numbers showing on the gauge? :-k

.

 

[LAB3]

You might find this link helpful, it automatically calculates all the parameters between differing tire sizes by entering the sizes. https://www.tacomaworld.com/tirecalc

 

[Sam 78 GS750]

This article here offers a pretty good explanation ----->https://www.thrillist.com/cars/your...accuracy-in-german-american-and-japanese-cars

It's talking about cars, but the same principles apply. Basically tire size, tread wear, over/underinflation, a number of other factors can all affect speedo accuracy. Across the speed range, the difference between actual and measured speed will be inconsistent. It may be 2 MPH off at 30 MPH, and 10 MPH off at 100 MPH.

Having a speedo that reads less than your actual speed could cause legal issues for the manufacturers, so they always err on the side of caution and design them to read faster than actual speed.

 

[Guest]

We suspect that dpeedos were intentionally inaccurate from the factory. Joe would say "my bike (with an accurate speedo) goes 105 mph". Bob says "my bike (with an inaccurate speedo) goes 110 mph". Guess who has the "faster" bike? Guess who has bragging rights, even though the bike might be slower, due to the wrong numbers showing on the gauge? :-k

Still a common complaint, even on modern bikes. My wife’s 2015 Honda CBR300RA speedometer reads about 10% high.

 

[Kingjoel]

Thanks to all that replied. I was hoping there may be a solution with the speedo gear on the hub. Tomorrow I will look into my GPS being off (I never considered it). I hope it is because it shows my F 150 off 4 mph also. I own seven bikes (all Jap) and most of the speedos are off about 4 mph, I thought 10 mph was extreme.

 

[roeme]

Many jurisdictions even require speedos to provide a higher reading.

 

[Downs]

78 GS1k here as well. 10mph fast.

 

[alke46]

79 GS 1000 here reads 8 mph fast.

 

[LAB3]

One more thing here, I've been using GPS since the days when it was still being called an experiment. Civilian GPS is relative, meaning that at any two given points your position is not dead nuts accurate but the distance between them is. The BEST way to test a speedo is to measure a distance and use a timer while traveling at a set speed to compute the actual speed.

 

[Steve]

One more thing here, I've been using GPS since the days when it was still being called an experiment. Civilian GPS is relative, meaning that at any two given points your position is not dead nuts accurate but the distance between them is. The BEST way to test a speedo is to measure a distance and use a timer while traveling at a set speed to compute the actual speed.

I have not been using one quite that long, but also have extensive experience. I agree that speed between any two points is pretty much "hit or miss", but the overall trend is rather accurate.

I upload my tracks every week to keep track of payable miles for work. It is sometimes interesting to look at the numbers between two points. One time, it showed my doing about 5 mph from where I had parked at the garage down to the street. It then showed me turning left and within 5 seconds, I was doing 138 mph. Of course, it then showed me doing 20, 25, 37 then 45 mph between the next few points. I knew my minivan was pretty quick, but did not realize just how quick it was. A previous van also took me to over 2000 mph while going around Indianapolis several years ago. Not only did it show me going that fast, but I jumped from my location on the freeway (elevation about 900) to a point several miles away and over 4000 feet up and back in just a matter of a few seconds.

GPS calculates position based on differences in a standardized time signal. It does not take much of an error in rounding to put you quite a distance away from actual position, but overall, when it says on the display that you are going 60 mph, you are easily within 1 mph of 60. The spikes that happen due to miscalculations just don't have time to show on the display.

.

 

[postman_pat]

Thanks to all that replied. I was hoping there may be a solution with the speedo gear on the hub. Tomorrow I will look into my GPS being off (I never considered it). I hope it is because it shows my F 150 off 4 mph also. I own seven bikes (all Jap) and most of the speedos are off about 4 mph, I thought 10 mph was extreme.

It is also possible that the return spring inside the speedo is losing strength or binding (?). Over time, this will cause your speedo to increasingly over-read speeds.

 

[Guest]

Last year I took my Speedo apart cos it was making a weird buzzing noise & jumping around, after oiling the internals( which cured it), I put it back together & found I'd foolishly put the needle back without zeroing it & now it's 13mph fast, I'm gonna leave it cos it's quite a difficult operation ,even thou it's annoying. Maybe you can do the same but do it properly.

 

[hillsy]

Just to play devils' advocate here - how do you know your GPS is correct?

:-k

 

[posplayr]

Just to play devils' advocate here - how do you know your GPS is correct?

:-k

Because gps in much more accurate than a gs speedo.

 

[hillsy]

Because gps in much more accurate than a gs speedo.

I would agree, however, how do you know?

It's funny how we just accept the GPS must be right?

 

[posplayr]

I would agree, however, how do you know?

It's funny how we just accept the GPS must be right?

Because a GPS receiver has several "Figures of Merit" that can be used to qualify the receiver performance above and beyond "Built-in Test" capabilities. The overall GPS system includes both Space and Ground segment which operate in a closed-loop fashion to provide the required Satelite pseudo-range data and ephemeris corrections. In a nutshell the system can check itself and if it is down you will not get a display from your GPS or you will be warned of inaccuracy.

If you consider the complexity of the system, your same questions could just as easily be asked of the cell telephone system. How do I know I'm getting an accurate voice from the person I'm calling? In fact, the voices you hear over a cell phone are more than likely to be much more in error (pitch) that a GPS. This is a qualitative comparison, but as I said the first time a GPS is so accurate compared to a GS speedo, that if it is out they will not report.


GPS doppler based velocity is about 0.25 m/sec or 0.56 mph RMS