Safe Speed Forums

I run quite an old car 1987 vintage, kept in first class mechanical condition by myself.

After purchasing my first map type gps last week I checked the speedo for accuracy and noticed quite a bit of under reading, 5mph slow at 30, 40 & 50mph.

What I'd like to know are two things please:

1. How accurate is gps speed indication with a good satellite fix?
2. What do the police & speed camera partnerships use to calibrate their kit?


Thanks in advance........Firefly

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More speed cameras = More carnage

I'm not familiar with the Navman but most GPS sets are pretty accurate on speed. Also, at 1987, your car isn't really that old - does it have an electronic or a mechanical speedo (or a hybrid)?

Some GPS units have "velocity averaging" which you can enable or disable. Without this, the deliberately degraded signal from the American military satellites causes the velocity shown to fluctuate a little. This effect gets less as speed increases. Also worth making sure the car was going in a pretty straight line at the time of the measurements. (If you drove round and round a tight enough mini roundabout, your velocity could show up as zero - or close to it).

Finally, (apologies if stating the obvious!) are you sure the set was displaying statute miles / hour and not nautical miles / hour and is the car on standard tyres? It is very unusual for a standard car speedo to under-read!

Anyway, none of this answers your question! As far as I'm aware, it isn't easy to re-calibrate a speedo - especially a mechanical one. It's probably easier to see if the manufacturer offered different final drive ratios and / or tyres. IF so, they would probably have a selection of speedo drive gears that just plug into the box. Just choose one that's closest to the true speed (but on the right side of "legal")!

Selective Availability (the "degradation" you mention) was turned off on May 1, 2000

http://en.wikipedia.org/wiki/Gps

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COAST Not just somewhere to keep a beach.

A young loner on a crusade to champion the cause of the innocent, the helpless, the powerless, in a world of criminals who operate above the law.

Thanks Mole & Handy.

The car speedo is fully mechanical readout, gearing & drive.
Manufacturer is Landrover.

I'm not going to bother to get it adjusted, what's the point, I can compensate for the error.

Next job though is to check the motorbike speedo with the gps in the tankbag window.

Great little unit the F20, very cheap at £113, great display & very sensitive. Oh and the speed camera warnings are a nice touch too.

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More speed cameras = More carnage

With any Landrover there are hundreds of possible combinations of tyre size/final drive ratio/gearbox ratios/overdrive ratio/speedo drive gear ratio/speedo head ratio and it could take ages to check each component to find where the fault is.

Getting a mechanical speedo recalibrated is simple.

http://www.speedycables.com/

Or do what I did. remove the speedo, paint over the existing markings, calibrate it by means of GPS or by following a friend and repaint the numbers in the right place.
I also have the fuel guage calibrated in 5 litre increments by a similar method.

I never knew you could calibrate a fuel gauge with a GPS!

Ah, I'd forgotten they'd stopped messing about with the signal! (not sure I ever knew, come to think of it)! That said, my hand held one still fluctuates slightly in displayed position when I stand in the same spot!

Just one more thought Firefly, have you got the right wheels & tyres on it? Years ago I had a LWB Series III and everyone told me to put Range Rover wheels & tyres on it to get a bit more ground clearance and improve the gearing for some better fuel consumption so it must have been a common thing to do.

Fisherman is right though. There will be dozens of speedo drive gears and I doubt they'd be very difficult to chnage on a Landy.

landrover fuel guage callibrated in smilies

=just filled up




= empty

on my old fiesta I changed the speedo to match the tyres by changing a small plastic worm gear on the top of the gearbox. take the speedo cable off, remove a spring clip take the gear out put the new one in. the problem is choosing the right worm gear.

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Speed limit sign radio interview. TV Snap Unhappy
“It has never been the rule in this country – I hope it never will be - that suspected criminal offences must automatically be the subject of prosecution” He added that there should be a prosecution: “wherever it appears that the offence or the circumstances of its commission is or are of such a character that a prosecution in respect thereof is required in the public interest”
This approach has been endorsed by Attorney General ever since 1951. CPS Code

When checked against GPS, my last car's speedo show about 7MPH difference comparing when my rear tyres were brand new and worn out!! Tyre pressure counts too!!

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If I have to explain you won't understand.