GPS compass speedometer


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Garybear
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Joined: Sat Aug 10, 2019 3:08 pm
Location: Stanley, Iowa
Motorcycle: 1987 GL1200 Aspencade

GPS compass speedometer

Post by Garybear »



I drive an 87 Aspencade and as many have discovered, the speedometer is off about 5 MPH at highway speeds and a magnetic compass can’t compete with the speakers magnets. Has anyone tried something like a GPS speedometer, compass like the Eling universal speedometer compass (https://www.amazon.com/ELING-DigitalGPS ... B07KP7HGJ4)?


Solo So Long
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Re: GPS compass speedometer

Post by Solo So Long »

There is actually no Federal law regulating speedometer accuracy on motorcycles (there is only one for commercial vehicles, 4% tolerance across the entire range). Most errors are a faster indication.

For about 40 years, US and Japanese motor vehicle makers have followed voluntary standards covering ELECTRONIC speedos (currently -- no pun intended -- SAE J1226).

A GPS speedo can't keep you out of a citation, but it CAN put you INTO one. If Officer Earn-While-You-Learn decides that your 8-miles-over is worth taking him out of service for 10 minutes, chases you down and pulls you over, you can carry that citation into court with a speedo check showing that it is inaccurate, giving you the ability to say that you thought you were within the legal limit.

More likely what will happen is that the cop will do a quick assessment, run your info, and decide that he's had the desired effect on your riding, letting you go with the lecture.

HOWEVER, if he spots your GPS speedo (or your GPS navigation operating), that notation on the citation pretty much guarantees conviction, because you had a "known-better" standard to determine your speed. He can write the cite, knowing that it will be upheld, especially if it's a LIDAR or radar stop. The assumption is that YOUR electronics should have showed the same speed as HIS electronics.

The annoying thing about this is that a GPS really isn't an accurate way to determine speed, moment by moment. Averages, yes, but imagine that your GPS hunts for position from 12 satellites, with an accuracy of 3 meters (accuracy changes constantly, based on signal strength). Now let's say you glance at the GPS every 5 seconds -- but see it while one satellite is saying that you're 10 feet ahead of your actual position, another is saying 10 feet behind. The speed calculation that your GPS reports averages between the positions, so one result may have you an extra 20 feet ahead, then a tenth of a second later may be based on you falling back 20 feet. Depending on which microsecond the GPS polls for its report, you may show a corresponding higher or lower speed, and you never know which. Now, let's say there is a similar jump each time you glance at the GPS, thus it seems that your speed is steady and legal (or, realistically, at the speed you think you won't get cited for), when you're actually up in the "Yeah, we'll stop him" range.

The GPS version of "Carol of the Bells" is having several units operating at the same time, each set to chime at the same speed, then you hit the road and set the cruise control for that speed. As they hunt back and forth, you will hear the ones which are more prone to inaccurate reporting.

The only time I trust a GPS speed read is for calibrating. I get up to posted speed, reset the GPS, then hold at posted speed on the speedo for a mile or so. If the speedo says 70, and the GPS says that my average speed has been 66, then I can make the correction mentally. After a little tire wear and maybe some air added, I do the same thing again.

Yeah, this was a really long way to say I wouldn't put on that GPS speedo, wasn't it?
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offcenter
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Re: GPS compass speedometer

Post by offcenter »

Garybear wrote: Mon Dec 14, 2020 9:05 am I drive an 87 Aspencade and as many have discovered, the speedometer is off about 5 MPH at highway speeds and a magnetic compass can’t compete with the speakers magnets. Has anyone tried something like a GPS speedometer, compass like the Eling universal speedometer compass (https://www.amazon.com/ELING-DigitalGPS ... B07KP7HGJ4)?
The reviews on that thing are awful.
George in Jersey.
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AZgl1800
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Re: GPS compass speedometer

Post by AZgl1800 »

Solo So Long wrote: Mon Dec 14, 2020 2:04 pm

The annoying thing about this is that a GPS really isn't an accurate way to determine speed, moment by moment. Averages, yes, but imagine that your GPS hunts for position from 12 satellites, with an accuracy of 3 meters (accuracy changes constantly, based on signal strength). Now let's say you glance at the GPS every 5 seconds --
I beg to differ on this:

I have compared my cellphone's GPS speedo reports to my Garmin 2797, and they are rarely more than 1 digit in difference..... e.g. 75 mph vs 74 mph....

if you are pushing +12 mph, you deserve the citation.

I have more 5 Garmin GPS units, and none of them have ever flickered as much as you describe.
They do give an average over the last 2 or 3 seconds, to that I agree.

But, most of the time, the Error is 3 meters on both the phone and the GPS units.


~John

'02 GL1800
2009 Piaggio MP3 250cc
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