Speedo accuracy

[LeeR]

How accurate are speedos supposed to be?

I took my GPS with me on the bike today and although 30mph is 30mph apparently 90mph is only 78mph.

I reckon my indicated top speed was around 95mph and yet the GPS said my top speed over the entire run was 84.8mph

Is this usual for a Suzuki speedo?
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[nunswimming]

Legally speedos are only allowed to over read by 10% plus an extra 2mph.

So essentially when you think you are doing ton you could in fact only be doing 88mph.

However obviously not all speedos under read this much people have put their bikes on dyno and the indicated speed is very close to the real speed.

Have heard it can depend on make of bike, type of speedos etc.

[Kickstart]

Hi

Depends how accurate your GPS is! Most GPS units are not that accurate, while performance ones don't directly use the GPS directly but instead work off the doppler effect on the GPS signal. Don't rely on a "highest speed" reading because that could just be a blip. For example I have had a highest speed reading of over 70mph on my YSR50 (does about 50mph really), but checking the logs showed that the speeds a second either side of the 70mph reading were FAR lower.

Speedos are allowed to be 10% +8kmh out for production but must read accurately or high. It cannot read low. Think nunswimming is getting confused with the guidelines for doing someone for speeding. They are also only tested at 2 speeds (from memory 50kmh and 120kmh).

On my 1200 Bandit the last marking on the speedo is 170mph, which coincides with the red line in top on standard gearing. It is an actual 154.5mph.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[davthegreat]

hi

ive seen people buy a bicycle computer and put it on there motorbike to compare speeds with normal speedo. how does this work because when you get one you need to input the wheel size and most of the computers have a lowest value of 20" ive seen some that say fit all bike though so they or is it just to the lowest value. Also another factor of the the calibration/setting up the computer to make it accurate is entering the tyre width wtf how can you enter for example 100/70 x 17 on my derbi senda into it when its made for bicycle tyres. Here is the sizes that can be entered into a random one i found on google

Schwinn Bike Computer Wheel Size numbers
Wheel Diameter Number
20" 1596
22" 1759
24" 1916
26" (650A) 2073
26.5" (Tubular) 2117
26.6" (700x25C) 2124
26.8" (700x28C) 2136
27" (700x32C) 2155
28" (700B) 2237

w/ tire
ATB 24X1.75 1888
ATB 26x1.4 1995
ATB 26x1.5 2030
ATB 26x1.75 2045
ATB 23x2 (650B) 2099
27x1 2136
27x1 1/4 2155

thanks if anyone can help people put them on so there must be a way of getting round it or getting a universal one.

[Smooth]

[Bikeless]

I used to run a bicycle computer on mine and you could put the front wheel diameter in,measured with a piece of string sat in the middle of the tread.

[davthegreat]

i see, ok thanks for the help

[davthegreat]

ok i like them sigma ones heres one on ebay


Item number: 260365354483 just copy and paste the number into ebay.co.uk for the item (for ppl who dont know what to do with the number)


do you know if it has a clock on it like 12hr/24hr clock, as in a clock not speedo clock. need a clock on my bike and the ones on ebay made for handlebars are analog wouldnt look good on my bike, it says clock in the writing but im not sure if its talking about the speedo when it says clock in the writing

also the one on ebay says in the writing 12mph = 24kmh but does the actual speedo on it that will work in mph

thanks for any help (assuming its the same as yours)

[Charlie]

I wouldn't go off highest speed either, I once got over 1000mph while on a walk.

We downloaded the log onto the computer, there was a glitch for around a second in which we visited the north poll and back
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Past: Honda x8rs, Honda City fly, Honda Hornet 250, Honda VFR750, Yamaha xt600e.
Current: Honda CBR929RR & Yamaha XT660Z Tenere

[T0MMY]

When my car has 130 on the speedo the GPS reads 116. Never tested the bike.

I was under the impression the GPS was pretty accurate, is that not the case then?
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[Moonie]

OT, how accurate are the roadside signs that tell you your speed?

I haven't got the GPS speedo any more and want to know how fast the chinkmobile really is.
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[Kickstart]

Hi

General use GPS is accurate, but it is designed to give position rather than speed. If it is a unit that takes speed by checking the distance between positions then a minor error in one of those positions (maybe due to the signal from a satellite being obscured) could make a fairly large difference to speed.

For example most GPS units take positions every second. If one of the positions is just 5m out then it could add ~11mph to the speed (could also subtract it, depends on the direction it is out) for that second. A crude display might just hide this by smoothing out the speed readings so you might not see it except by checking the log.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[LeeR]

I take your point Keith, and thanks everyone for the answers.

Having thought about it some more, I reckon that given a GPS tolerance +/-3m then at speed I'm obviously going to get some anomalous readings.

I have noticed that when I'm in 50 zone and I've slowed down to 50mph that all the vehicles seem to be moving at a fairly constant speed.

I don't reckon my speedo is out, but I do think as has been pointed out that all speedos report a higher reading and they seem to be consistent.
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[Kickstart]

Hi

Not sure they are always consistant. Certainly the accuracy of the speedo in one of our cars varies when compared to GPS. Much of the time it is 5mph high (goes up slightly at high speed, but not much, but 35mph indicated is 30mph, while maybe 86~87mph indicated is 80mph), but after a long journey it strangely gets more accuate. Had 2 GPS units in the car at once (one normal bluetooth one with a PDA, the other a GPS performance meter taking 10 samples a second) to double check it and it definatly gets more accurate (will land up overreading by 1~2mph at 30mph).

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Smooth]

[sagalout]

When we did a day at Bruntingthorpe, the datalogger said 165.5 mph, the tomtom GPS said 165mph and the clocks read 185mph lol.

Most bikes I've had have been about 10% out.
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[TimNorwich]

I think lower down speedo's are accurate, not so much the higher the speed.
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[binge]

[T0MMY]

[davthegreat]

thanks binge youve cleared it for me, i think ill do your idea of the full rotation of the wheel mines a 17inch so if yours works with a 12" itll defo work with a 17" thanks for your help