Legality of GPS as a speedo

Good evening all.

I understand that a speedo demonstrates how fast the front wheel is
spinning and that a normal push-bike speedo is MoT-legal on a
motorbike.

As a GPS unit does not calculate the mph from the wheel spinning,
would that mean that one would not be legal as a replacement speedo?

Just curious.

 

I'd imagine they're legal whenever they have a lock. GPS speedos are
expensive though... http://www.livorsi.com/catalog/gps.htm . You're best
off buying a cheap gps unit.

 

I didn't think that you actually need a speedo for MOT. I'm pretty sure
I've never seen one tested.

Well, none of my cars calculate speed like that so that could be your
answer.....

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I was curious because I just fitted a handlebar mount for my old
yellow Garmin eTrex (will come in handy one day for the general
direction I should be travelling in while lost) and wondered if it
would "do" as a make shift speedo. My normal speedo works perfectly.

I got it up to an 'indicated' 92mph and after checking my maximum
speed at the end of my 2 hour bimble[1] the eTrex stated a max speed
of 82.4mph

[1]I only intended to go out for ten minutes to see if the mount was
secure while zooming but the Hooning Bug took me over and I wasn't
strong enough to resist. Hmm, lovely quiet twisty fast country roads
with the sun setting in the background.

 

I'm sure they're not.

I've had cars and bikes pass with non-functioning speedos.

 

Is it a proper motorcycle mount or a pedal bike mount? If it's a pedal
bike mount, it'll destroy your GPS, apparently.

Heh.. my Bandit speedo is only about 4 mph out, which is good enough. My
CG125 speedo on the other hand, varies between 4 mph out and 12mph out.

Great init.

 

Garmin just list it as a "handlebar mount" - hopefully it won't
destroy the GPSr, but if I does it doesn't matter, when I upgrade my
bike/licence/both I intend to get a proper motorbike GPS unit.

I did eight or ten laps of a lovely big roundabout just because it was
lovely and empty, getting a bit further over each time. I decided I
should go and play somewhere else and moved to a different roundabout
then pissed off home when a police car turned up and also did two laps
with me

 

One problem with GPS is that it only tells you what direction you
are *moving*; if you're stood still you have no direction data. I
bought a cheap digital compass at Aldi the other day (£10) that might
solve such an impasse. However, I have to solve one little problem
first, apparently it shuts down after two minutes which is not so good
for hands-off operation.

Which means I'll need to pull it apart; I'd assumed it was just
a compass needle in an encoded disk but if it needs to shut off to conserve
power maybe it's really a hall-effect field detector. That'd be _very_ cool
at the price (may explain the weird "recalibration" procedure, too).

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Don't know if it affects legality, but a GPS only works when it can
see enough satellites for long enough, which is not always true when
passing between tall building or in tree lined avenues.

 

They're not a testable item.

 

My Garmin 60CS GPS has a built in magnetic compass that kicks in if you
go below a programable speed for a programable amount of time. It's also
got a 'take me to the local / shopping / petrol / restaurant / hospital
/ historic / other place function. It's also got functions for aircraft
/ walking / driving / boating / hunting, it's waterproof and it floats.
Very much better than these limited PDA based jobbies... although one
limit is that it needs to be held flat to use the compass, but there are
dedicated GPS out there with a magnetic compass that doesn't need to be
held flat.

 

This is where it may fall down. The constructions and use states [1]:

"...

1. The vehicle shall be fitted with a speedometer capable of indicating
speed in mph at uniform intervals not exceeding 20 mph at all speeds up
to the design speed of the vehicle and capable of being read by the
driver at all times of the day or night.

2. For all true speeds up to the design speed of the vehicle, the true
speed shall not exceed the indicated speed.

3. For all true speeds of between 25 mph and 70 mph (or the design speed
if lower), the difference between the indicated speed and the true speed
shall not exceed -

V/10+6.25 mph

where V= the true speed of the vehicle in mph.

..."


http://www.dft.gov.uk/stellent/groups/dft_roads/documents/page/dft_roads_5
05075-02.hcsp

The key phrase is "at uniform intervals" which if the GPS loses line of
sight it wouldn't be working.


[1] Worth noting this quote comes from "Schedule 2 Vehicles"
(imports, home builds etc.) of the SVA regulations but I can't
see it being different [2].

[2] It was different in 1985 when I was nicked for no speedo (when I
brilliantly used "speedo doesn't work" to explain my speeding in
a 30) and I managed to have that charge withdrawn.

Pity amateur Rumpole didn't get the other seven (IIRC) charges
withdrawn.

 

So my CG is just within spec at 70mph, which gives me that warm fluffy
feeling inside.