Speedo on my bike is incorrect?

[vifferman]

My bike is geared down, so the speedo over-reads by about 11%. I did have it bang on with an electronic speedo corrector fitted, but then my odometer was under-reading by ~6% . I was going to check and adjust the corrector by asking a cop if he could 'zap' me and then tell me my actual speed, but then we bought a TomTom so I used that instead.
The car (Peugeot) reads 110km/hr at 107 true, or 100 at a true 98km/h, so it's pretty accurate for a car.
Apart from when the speedo stops working altogether, which it does very frequently (like on the way to the testing station the other day - didn't work the whole way, then mysteriously came back to life during the test).

[CookMySock]

....when the speedo stops working altogether, which it does very frequently (like on the way to the testing station the other day - didn't work the whole way, then mysteriously came back to life during the test).

It's fucking with your mind.

My speedo indicates high, so I just crackle along and 120 clicks indicated and enjoy myself. I find it quite satisfying watching it say "120". I can even zip past cops and give them a wave - glance down at speedo, "120", niiiiiice, fuck I showed him.

Farken arse, I took that corner at 120k, fuck I'm a god.

Problem? Where?

Steve

[aroberts]

I used my GPS and found the speedo out quite a lot. So I stuck little waterproof dots on the speedo to mark 50, 80, and 100km/h. Works for me.

[thealmightytaco]

I havent had a vehicle yet that wasn't about 20% out, 60 in a 50, 85 in a 70, and 120 in a 100k zone in/on all of them, Nissan stationwagon, Mitsi van, a Honda NSR, and 3 Suzuki's (FXR, GSXR, DRZ). It's a conspiracy I tells ya.

[taff1954]

On a new front tyre, the GPZ was an average of 6% out - based on the 'your speed is' trailer. As the tyre's worn, the error has crept up, so it's now around 8 -9%, so there's another factor to consider, if your speedo is driven from the front hub.

[Mikkel]

I used my GPS and found the speedo out quite a lot. So I stuck little waterproof dots on the speedo to mark 50, 80, and 100km/h. Works for me.

A GPS is the way to go. I wouldn't necessarily bet my money on the "Your Speed" signs being any more precise than your own speedometer. If you ride the margins, speedwise, then you wouldn't want to take that chance.

Also, the deviation will be a linear deviation. As such there will most likely be a proportional deviation (i.e. a percentage of the indicated speed), however it is not unlikely that there is an offset, a constant deviation, as well. This is the reason that the police cars need to have their speedometer calibrated every once in a while, every 6 months IIRC.
Almost all speedometers read too high. I haven't had any cops as much as raise an eyebrow going around town at 60 km/h indicated. If you get hold of a GPS unit just log your actual speed for 50 and 100 km/h indicated and put it in an excel spreadsheet. Plot the real speed against the indicated speed and you can add a trend-line to give you equation. The more data points you log the more precise your fit will be. If it has any interest I'm happy to flick a spreadsheet together for you, then you only have to do the measurements and enter the data.

[hellokitty]

I got told off when I did my restricted for going too slow - 40kms when my speedo said 50kms. I got really stressed when i did my full license but the guy testing me said I could speed, as all bike speedos are inaccurate. He just said don't go too slow. So I went balls out on the open road (rural part of East Coast Road) and 10 minutes into the test while I was still in the warm up time, he passed me - said I could handle the bike really well! I asked if he wanted to see me in traffic etc and he said there was no need! Weird!!!!!!

[Pixie]

Most bikes and quite a few cars have offset speedometers. I always thought the "your speed" signs were wrong but recently somebody that has ties with the police told me that all of them are very accurately calibrated,

Not true: the pole mounted ones in Auckland (on Mt Albert Rd and elsewhere) read high.

[fliplid]

Wearing a fluro vest seems to mess the reading up on the "Your Speed" signs. Been through the one heading towards puke with and without a hi viz on, and at the same indicated speed, and it seems to make the sign reading jump between different readings.

[Chooky]

Yer... just zip tie your Garmin GPS to the handlebars and then spend a bit of time working out how far out the speedo is.
Put some twink marks on the speedo face and it'll all be good.

[Bass]

The "your speed is" signs told me that the Trumpy speedo was about 10% optimistic at around town speeds - no surprises there.
However, the cop with the laser and the ticket he gave me, told me that it improves to only 3% optimistic at 120 kph.
Bugger!
Oh well, at least I know now.

[quickbuck]

Wearing a fluro vest seems to mess the reading up on the "Your Speed" signs. Been through the one heading towards puke with and without a hi viz on, and at the same indicated speed, and it seems to make the sign reading jump between different readings.

Yeah... May be that's why they always say "Slow Down" when i pass them

[varminter]

Yup, the one's around here in Bloataville say, 'slow down' at 2kph over, shame I'd like to know for sure.