Used GPS to compare my speedo yesterday...

....found out my speedo is off almost 5 mpg at 60 mph. Reads 60, GPS confirms 55/56.

Is this common or have I had a sprocket mod done by a PO? Tach is just over 4k at 60 mph.

Incidentally, I got 54 mpg on a 140 mile trip yesterday (mostly interstate); corrected by 8% that means I got 50 mpg?

 

Apparently (I have no proof whatsoever) the speedo over reads by about 10% but the odo is correct.

 

Its common.

I am not sure about the USA, but in the EU there is a legal reason why an exaggerated reading is normal, indeed its pretty much standard on almost all road vehicles. By law the speedos fitted by vehicle manufacturers must never understate the true speed, hence speedos fitted to vehicles are generally allowed to read up to 10% over.

If you are really worried about knowing the true speed, then your GPS should give a better fix. There is also a thing called a speedo healer which you can use to correct the speedo on the VFR to show the real speed. However this will then require adjustment from time to time, as tyre(tire) wear will change the rolling radius, likewise changes in tyre(tire) pressure will also skew the displayed speed.

By comparison my GPS shows 110kph when the VFR speedo reads 117kph.


Hope that helps



SkiMad

 

Yeah, completely normal. Around 5%-8% is average, but your odometer is most likely dead on. I put a bike computer on mine just for peace of mind when running through likely looking speed traps.

 

I am surprised that the odo is right on, as the odo is usually directly proportional to the speedo. But I have no experience with this so I will take your word.

My speedo is out by about the same as what has been mentioned here already.

If you think about this for a moment, the speedo in a car is calibrated to reflect a speed based on the number of revolutions of a wheel over a certain distance. Measure the outside circumference of a wheel, use a formula and you can get an accurate speed. Thing is, a car has four wheels. They all stay relatively flat on the road surface. On a bike, when you are cornering, your tire is making contact somewhat either left or right of the center of the tire. So, there are more revolutions of the tire to cover the same distance that if you were driving straight. That would make your speedo inaccurate.

If you don't have a life and really want to see the difference, go out and measure the circumference of the tire at it's longest point, and the circumference say midway between the center and the outer ridge. Then do the math. The difference is quite noticeable. Same thing when you change the tire size on your car.

 

I can't see how anyone can claim a speedo as off but odometer is correct. They are relative & the source (the gearbox sprocket nut) is not constantly accurate due to tyre wear. This is why speedo's over read, as they are set to be close to accurate when the tyres are new & fully inflated. Thus the odo & speedo have an inherent inaccuracy, that gets worse as the tyres wear down. The accuracy of each may be different, but the percentage chnage with wear will be the same for both.

As an example, if a we assume a new tyre as 25" diameter = 78.5" circumference x 1000revolutions= 1.2389miles, so the tyre wears say 1/4" of tread reducing the diameter to 24.5" = 76.93" circumference x 1000revolutions= 1.2141miles. So that's a 2% change in accuracy, or a gain of 2 miles per 98miles travelled. 20 per 1000, 200 per 10,000 or 1000, per 50K miles ! And that is the actual inaccuracy for real, never mind that most speedo's read high even on new tyres, so if we say it's 6% as an average, that's an over read of 3000miles at 50K miles, i.e. it's only travelled a real 47K miles !

Since I fitted my speedo healer & GPS verified its accuracy, I have lost 7% mpg, but the reality is its now recording real mph & mpg 36mpg

 

The odometer is driven by gears from the speedometer cable, while the speedo is moved by magnetic force, and not directly attached.

I went through one of those YOUR SPEED IS things the other day, it said I was doing 56 (in a 55 zone) while my speedo read 69 or 70.
So I'll have to break out the GPS as well.

 

Don't believe it if you don't want to, but the fact of the matter is that the odometer is correct and the speedo is not. Facts is facts. Having had many, many motorcycles and all but a couple were that way. And all the Japanese bikes were that way. I've never come across a bike that had an odometer that was off by as much as the speedometer was.

 

Those "your speed is" devices are calculated using radar, which is only accurate to plus or minus 2 km over a 100 km range. They are not precise either. Then again, GPS has inherent errors in that it does take time for the signal to travel from the device, to the satellite, then to return to the device. There is a time delay. What effect that has on the GPS speed, I don't know. I suggest it is absolutely minimal. Even with laser, there is a cosign error unless the laser is directly in front of the oncoming vehicle, which is not a good place to remain. I believe that GPS is likely to be your most accurate measure of speed shy of a stop watch. Then even with that, there is reaction time of the operator of the stop watch that will affect the outcome.

Where were we now?

 

Ouch!!! My head hurts!!! Carry the one and divide by purple....
So in english....Um yes???

 

I was curious about my speed too so I bought a GPS speedometer for my phone. My speedometer said I was going 130 mph but the GPS said a was only going 117 mph. So to be off by 13 mph is quite a bit.

 

Seems reasonable, thats 10%. Remember the higher the first number is, the bigger the discrepancy will be (maybe I should have stuck with maths at school after all)

 

Strangely, the speedo is NOT moved by magnetic force, unless you have a cable driven speedo ! The 5th gen & all later bikes use an electronic sensor on the gearbox sprocket, that can ONLY measure the number of revolutions & a stepper motor uses that as an input to swing the needle or on 6th gens etc, an algorithm turns it into a digital reading !

All odometer have the tyre wear error, it's impossible to compensate for, unless you know of a bike with a GPS reference sensor built in that auto adjusts the odo input ratio to compensate for wear !?

There is NO communication between a GPS & any Satelites, its a purely receiving system only. The satelites send out a time coded signal continuously & the receiver, compares the time signals & time interval between signals from 4 to 16 satelites (depending on the receiver & satelites visable above the horizon) then calculates its position by 3D triangulation & thus can calculate distance travelled over time to generate a speed reading. If you have a high speed receiver, then it will update every 10th of a second, so the accuracy is quite good. But some cheap units do NOT take vertical drop into the equation, so speed over the ground can be more than the horizonal distance speed calculated by the GPS, so always speed test on fairly level ground.

 

My '04 is off my about 9% (and this is normal). So, I got a speedohealer which intercepts the signal between the sender and dashboard. Now my speedometer agrees with GPS. One note though... now my odometer under reports by 9% So there's a tradeoff. I just add the 9% to odo readings for oil changes, fuel mileage, etc. I'd rather have the correct speed indicated while riding.

 

Yeah, I know... Old thread... But I'm pretty sure this is NOT how GPS'es calculate speed.

There is a time calculator built into the GPS device. All of the geosynchronous satellites are perfectly aligned in their clocks. As the signals are sent from the satellites and received by the GPS, a number of things are done with the signals inside the GPS...

- The time stamp for each satellite signal is compared to that of the other signals received. Based on pre-programmed data inside the GPS, it is able to use triangulation to calculate its exact point on Earth (that is then overlaid on a map)
- The time / clock of the GPS is corrected / set based on the timestamps of the satellites (the GPS can calculate the time based on delays from the different satellites and such).
- Moving speed can be calculated based on updates to the calculation of position

Consumer GPS devices can be off by as much as 10 feet in any direction. So, calculating your speed isn't perfect. But, it's normally fairly close as long as you're not creeping forward at 5 feet per minute (which is only slightly faster than HALF a mile per hour).

 

I will bow to that. I know very little about how GPS signals work. I do have a pretty good background in laser and radar speed meters though.

 

Just out of curiosity. I heard, from a non reliable source, that the consumer grade GPS units were purposely calibrated so that the position showing on the device was intentionally inaccurate for Security reasons. I am speaking of the very early hand held GPS that hikers and hunters and the like used to purchase when these first became available on the market. Back when they were a stupid high price. Do you have any insight into that?

 

From some research I did a couple of years ago, DoD (I believe) mandated that consumer grade GPS be manufactured to NOT be more precise than 10 feet / 3 meters. From what I understand, that mandate is no longer in effect and consumer grade devices can, in fact, be quite a bit more accurate.

I suspect that, for our everyday use purposes, anywhere up to 20-25 feet of accuracy is quite fine. We're trying to get from Florida to Oklahoma, not trace printed circuit boards! lol

 

Kinda what I was lead to believe. DOD was behind it. That's fine with me. If you are hiking in the bush and get lost, even 100 yard radius will get you found.

 

Agreed.

But, since we now use them to tell us when to make turns on public roadways, 10-20 feet is about the limit of what we can safely tolerate. Of course, that's assuming that the maps are up to date.