could my speedo be that far off?

[Rocky]

Yesterday, I went for a short ride and noticed one of those construction zone speed detectors so I sat my bike to the posted speed of 45 mph to see what would register and it showed 38 mph. I was the only vehicle on the road so I know it checked me. I can't imagine my speedo being off by more than 10 % at that low of a speed. I could see a mile or two but not 7. What do you think? I know those trailers are towed all over so I can't imagine them being to accurate either?

[Juice]

I rode through one of those near my house. I also register about 5-6 mph slower then the reading on my speedo. I think that my difference was due to gearing changes.

Is the bike stock? How many teeth are on the rear sprocket Dennis?

[Drivin&Cryin]

I don't believe your speedo is totally accurate. It may show only a few MPH off at 45 but at higher speeds it may be off even more. I would use a gps to figure out just how far off your speedo is at different speeds to figure out if you need to correct it. I use a speedohealer for mine and it works great. My speedo is accurate to the Tee and my mileage is only off .6 miles for every 10miles. I can live with that. I really dont care about the mileage aspect but it is a plus. I rather know how fast im going rather than distance. You can still calibrate it for mileage or you can do a compromise for both. I think it could be worth the investment getting a speedohealer rather than a ticket. A ticket may cost you as much or more than a speedohealer and its also so easy to install. May take 20min

[minig0d]

No bike's spedometers are completely accurate... and if you have changed your front sprocket your spedometer will read off by a large #... if you went down one tooth on your front sprocket it'll probably read about 45 when you are actually going 40 or so... if you want I have a little excel sheet I can post that you can punch in the specifics on your bike and it'll tell you all about your gearing and changes and all... lemme know if you want it and i'll upload it somewhere...

but also if your sprockets haven't changed most spedometers are off by 6-15 mph at 100mph+ but down in the 45mph range they should be fairly accurate (unless again you've changed your front sprocket which is where your bike picks up the measurements)... if you want to double check your exact speed, any gps unit should be able to give it to you extremely accurately...

[BOILERMAN]

am I the only one that doesn't really care if my speedo is off?

[scooterhose]

What bikes are we talkin here. Most bike speedo's are driven off the front wheel so gearing wouldn't matter. True?

The error in you speedo does seem excessive though.
None of my bikes are off by more than 2-3%.

Scooter

[minig0d]

What bikes are we talkin here. Most bike speedo's are driven off the front wheel so gearing wouldn't matter. True?
The error in you speedo does seem excessive though.
None of my bikes are off by more than 2-3%.
Scooter

not the front wheel the front sprocket now :)

[scooterhose]

Those bastards, next they'll be puttin the fuel pump in the,,the,,gas tank.:D


Scooter

[kenwelshons]

I got the busa up over 200 mph and the gps said...... 182 mph. The bikes Speedo said 210ish.
http://images4.fotki.com/v45/photos/3/302107/3319104/PB110928-vi.jpg

[scooterhose]

I've had my ZX up to about 185-190 indicated.:eek:

[Rocky]

Thanks guys. I'm going to pick up a GPS and get a more accurate reading.

[thedrizel]

dude, dont spend money on a gps unless you just have to have one. A $25 bike computer is more accurate than anything and takes up almost no space. Just make sure the one you purchase registers over 100mph. I forget the brand that I used to have but it registered up to 188mph and is set to the EXACT size of your tire so there is no error.

[kenwelshons]

except that the size of your tire gets smaller with wear....

So how often do you remeasure?

bike computers are a hassle with the magnet mounting and the pick-up. GPS if it is on the bike it's working.

[thedrizel]

except that the size of your tire gets smaller with wear....

So how often do you remeasure?

bike computers are a hassle with the magnet mounting and the pick-up. GPS if it is on the bike it's working.

Well condsidering it is adjustable down the the mm, if you wish to be that accurate you have got problems. When you wear off 4mm of circumfrence I doubt you actually care about EXACT speed. For that matter, the faster you go the more the circumfrence grows as well. Or maybe if you are a one or two psi low on the tire that will change the measurement as well. Speedometers are an AVERAGE anyway and not ment to be exact measurments.
Affordable, hand held GPS' are not that accurate anyway as the military allows only a predefined error built into the units. A complex logrithm is built into the guidence systems of military aircraft, VERY EXPENSIVE civilian aircraft units and missle guidence systems. The handheld units advertise to be accurate but that is only theroitical as our satalites are delibertly feeding misinformation in the intrest of National Security.........but hey. Spend your money on anything you wish, just don't argue with someone when they are trying to give alternatives.
Magnet placement takes about 10min and some forethought, but not everyone can do that.asdf

[scooterhose]

Well condsidering it is adjustable down the the mm, if you wish to be that accurate you have got problems. When you wear off 4mm of circumfrence I doubt you actually care about EXACT speed. For that matter, the faster you go the more the circumfrence grows as well. Or maybe if you are a one or two psi low on the tire that will change the measurement as well. Speedometers are an AVERAGE anyway and not ment to be exact measurments.
Affordable, hand held GPS' are not that accurate anyway as the military allows only a predefined error built into the units. A complex logrithm is built into the guidence systems of military aircraft, VERY EXPENSIVE civilian aircraft units and missle guidence systems. The handheld units advertise to be accurate but that is only theroitical as our satalites are delibertly feeding misinformation in the intrest of National Security.........but hey. Spend your money on anything you wish, just don't argue with someone when they are trying to give alternatives.
Magnet placement takes about 10min and some forethought, but not everyone can do that.asdf


I believe they did away with the built in error about 4-5 years ago.

[scooterhose]

OK longer than that..

http://www.eham.net/articles/638

[Katy]

yall wear speedos? ha-ha.

[thedrizel]

OK longer than that..

http://www.eham.net/articles/638


I did not know that. I have seen GPS in person be off a considerable amount within the past 6 months and just assumed it to be the "SA" error. I wonder how much truth is being told really.

[scooterhose]

I did not know that. I have seen GPS in person be off a considerable amount within the past 6 months and just assumed it to be the "SA" error. I wonder how much truth is being told really.


Hahaha...With Clinton? Probably not much. Now that you mention it, that article was prolly all BS.:)

[VM 2]

If you got a nextel gps enabled phone, you can also track your MPH from there.

[scooterhose]

If you got a nextel gps enabled phone, you can also track your MPH from there.


Dang ,still cameras, video cameras, text messaging, internet access,GPS, music,videos...Do the make phone call anymore?qwer

PS I thought all Nextel phones were gps enabled?

[kenwelshons]

dude, dont spend money on a gps unless you just have to have one. A $25 bike computer is more accurate than anything and takes up almost no space. Just make sure the one you purchase registers over 100mph. I forget the brand that I used to have but it registered up to 188mph and is set to the EXACT size of your tire so there is no error.

Man, I'm just hitting on you cuz you said ""EXACT"". And the truth is that a motorcycle tire has roughly 1 cm of tread. So a brand new 17" tire has about a 3.1% error between new and worn. Small but by no means exact.

Personally, I like to use my sextant to accurately calculate my speed.

Speed is not important when you're fast like the Juice.

And to VM 2... They started that so the cops could track drug dealers easier, duh...

[thedrizel]

whatever...apparently the speech police are writing tickets.

You want to get that technical on error b/w new and worn you better factor in a LOT more information. I'm not going to argue here because then we would both look like dickheads.

Most people understood my point, sorry you didn't.

[kajunKLR]

No bike's spedometers are completely accurate... and if you have changed your front sprocket your spedometer will read off by a large #... if you went down one tooth on your front sprocket it'll probably read about 45 when you are actually going 40 or so... if you want I have a little excel sheet I can post that you can punch in the specifics on your bike and it'll tell you all about your gearing and changes and all... lemme know if you want it and i'll upload it somewhere...

but also if your sprockets haven't changed most spedometers are off by 6-15 mph at 100mph+ but down in the 45mph range they should be fairly accurate (unless again you've changed your front sprocket which is where your bike picks up the measurements)... if you want to double check your exact speed, any gps unit should be able to give it to you extremely accurately...


am i missing something here? unless the speedo is connect to the tranny and not the front wheel, changing the sprockets won't do anything for calibration.
Or were you just fishing to see whos paying attention?

[minig0d]

its measured off a sensor that is off the front sprocket/sprocket shaft in most cases... any change in either front or rear sprocket will make the front sprocket turn a different number of revolutions per mile therefor throwing off the speedo...

[iridefar]

Speedometers are allowed by federal law to be up to 10% in error, but they can only read faster than actual. That is why when you think you are getting away with 70 mph in a 65 mph zone, you are not. You are probably going only 66 mph.

I don’t believe anything is more accurate than a modern 12 satellite GPS (other than the military units). While they do have a margin of error based on the number of satellites being hit, as long as that margin remains relatively the same, the speed (and distance) measurement is totally accurate. That is why the Iron Butt Association does not accept odometer readings for validating rides. They will always read too many miles. This applies to cars as well.

[CajunRider]

My GPS is usually accurate within about 10-15 feet of actualy location. Being that it's a "Global" positioning, I'd say that's plenty good enough.

Take the fact that a mile is 5280 feet... then the GPS speed accuracy should be within 0.2% of actual. That's if the 15 feet range changes very fast.

Plenty good enough for me.

Of course, if it doesn't find a good signal, my GPS has been off by 50 feet before. Of course, the GPS unit warned me that it would be off by 50 feet.

I'd say, the GPS is the most accurate measurement we can get for any reasonable amount of money. If nothing else... just borrow one! Come to Lafayette, I'll let you use mine for a few minutes. :)

[dewnmoon]

Most "speedos" will be inaccurate to the tune of 3-7% (generally they read high) unless you have a specially calibrated unit such as on an "Authority" bike.

You can test this by using a bicycle speedo such as a Sigma, Cateye or similar and calibrate it via wheele diameter or tire circumferance.

GPS: most that have a clear signal will be very accurate as well. SA selective availability was turned off some time ago. Suggest using them with the unit set on its highest sampling rate in an open area (outside city away from tall buildings and away from steep mountains and deep forested areas to avoid any possible signal disruption.
some report that accuracy may be more accurate at night as well.

[setacwoodtic]

My gixer said 189 on the clock with one up in front and one down in rear making actually read slower, so really I was doing just over 205 MPH on a 98 GSXR 600, light mods, LOL. No friggin way. Dont pay any attention to that dream machine on the bars, it's for kids who watch crap like fast and furious and biker boys yet have never seen a track let alone been on one. Also my KLR 650, (which is driven off the front wheel hub), is accurate under 30 mph and over 65mph, strange huh. Basically you should just get a GPS like are man with the BUSA did and never think about it again.

[cajun66]

Sounds kind of silly, but I drive by Tachometer over Speedometer because of how inaccurate it is.
40 reads as 45
50 reads as 56
60 reads as 68
70 reads as 80.

That's on the Aprilia. The old XL500R reads 5mph faster than actual speed. I think the needle is off.

[Ritten]

When Jason70360 and I went ride up to Red Bluff, he was amazed to find out how far off his speedo really was. He was under the impression that I was running the interstates at 90-95!! When we got back on the highway I showed him the GPS saying I was actually running 78-80 and at those speeds the Beemer was reading 2mph faster than the GPS. The Futura, I've come to find, reads about 5mph faster than actual at 80mph......not too bad.

[jason70360]

I dropped one tooth on my front sprocket on the 954rr, so I knew my speedo was off. But Ted is correct, I didn't think it was off that much. It reads 93 when going 80 (gps).