Speedometer vs. Android GPS App

[Snop Doog]

Whilst doing just under the limit for speed cameras I have found that other road users catch right up to me and the ones ahead of me just seem to disappear into the horizon. That raised my suspicion that my speedometer is dodgy. So I downloaded a speedometer app and according to that my bike's speedometer is out by around 5mph, but I don't know if I'm convinced.

Basically, what's more trustworthy, a 6 year old bike's speedometer or gps data via a mobile phone?

Thanks in advance

[LittleJ97]

Do you have the correct sized tires/wheels? Being an inch out could affect the calibration with the speedometer

[Snop Doog]

[barrkel]

Speedos always overestimate your speed.

Drivers generally have or have had the use of a GPS, giving a true straight-line constant speed for a given indicated speed. It's just as easy to hit a speed limit dead on with a GPS as with a speedo, but the GPS will be more accurate and you'll be going faster.

I have no idea how accurate your phone GPS is, because phones use a bunch of different mechanisms for detecting location and speed, depending on what's available and what will save battery. If the phone is locked on to GPS, it should be more accurate in a straight line for steady motion. A proper GPS satnav would be more reliably accurate. Still not accurate going around a corner though.
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[Snop Doog]

[MarJay]

GPS should be REALLY accurate as they use the Doppler effect to measure your speed rather than position sampling.
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British beauty: Triumph Street Triple R; Loony stroker: KR1S; Track fun: GSXR750 L1; Commuter Missile: GSX-S1000F; Cheap project: CBR900RR FireBlade
Remember kids, bikes aren't like lego. You can't easily take a part from one bike and then fit it to another.

[iooi]

Many people seem to rely on the fact that speedo's under read and will think that doing that bit over is Ok.
Or they have been through one of the growing number of speed reading signs now, and take that shown speed as being correct.

I'm personally not sure I would rely on GPS on a phone as too many things can effect it. From tree's blocking links to cpu being a bit slow.

Personally I would rather be a bit slow and make sure I do not get the ticket and let the rest worry about themselves....
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Just because my bike was A DIVVY, does not mean i am......

[Northern Monkey]

The GPS data on your phone will be very accurate, probably to within 1 or 2%. The speedo of your bike will nearly always over-read, normally around 5% IME.

I'd happily ride past a speed camera at the higher GPS speed
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Fisty: after polishing the tank with the glistenng beads of sweat from my full hot scrotum, I filled the headrace bearings with 10cc of my manmilk

[Doomsnite]

I been told that the po po give a 10% allowance due to differences in speedos so when going through cameras or average speed check, I will do 10% more.. e.g 55mph in a 50, 77 in a 70. Ive never been done yet.
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Triumph Street Triple-R
Sold: Honda Hornet CB600F-Y / Yamaha YBR125

[smegballs]

afaik push bike speedos, where you measure and input the exact rolling circumference are pretty damn accurate

[el_oso]

why is this even an issue?

So you have to back off a couple of mph over a 20 meter distance. Might make your total journey time about a second longer.
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[Albigularis]

[c-m]

Most phone GPS and most consumer dedicated gps devices only poll once per second, so when accelerating you'll probably see your speed jump in increments.

A high quality device will poll up to ten times a second so will be more accurate.

[B5234FT]

I normally go out in good weather with a decent GPS signal and drive on a straight road at a steady 70 and note the error. Repeat at 50 and then again at 30.

The rest of the time I drive based on the speedo (as it always reports, never runs out of battery or signal, but if I know its 5% under at 30 and 10% under at 70 you know you can be showing 77 for instance.

[Snop Doog]

Okay, thanks everyone!

Lesson learned: Speedometer is a liar and a thief. Don't give it too much trust.

Much appreciated

[iooi]

[Accuhaler]

[FretGrinder]

[Rogerborg]