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I heard a while ago that most car speedometers overestimate your speed by about 5-10% to discourage you from speeding. Now, my Google maps app has its own speedometer and I've noticed that when my car says I'm doing about 45, GM displays 40 roughly. Is it possible that GM is wrong, or is it definitely the car?
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I’d say it’s not to discourage you from speeding but to give themselves a lit of leeway in case a driver gets a speeding ticket and tries to sue the manufacturer. In addition, speedo accuracy changes as tyres wear, so allowance also has to be made for that.
I’ve checked speedos against multiple GPS apps, and I’d always go with GPS speed as being more accurate. Have a look when next doing 50mph (speedometer) in a 50 zone on the motorway how many trucks with accurately calibrated speedos come past you...
Incidentally, I’m a full time Google Maps user on iPhone, and have yet to see the speed display on screen (it’s there on Waze) - I’m wondering if it’s just an Android feature?
Edited by mcb100 on 31/07/2019 at 11:57
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My 07 Honda Civic digital speedo drastically over read by 4 mph -5 mph at 30 mph - from new, confirmed by owner forums detailing various electronic means of re-calibrating it ,"Speedo Healer" was one brand name I remember.
But my current Accord analogue speedo is spot-on accurate confirmed by a little gps speedo I use to keep the right side of french speed limits.
Remember, a gps speed readout is only 100% accurate on a level road when travelling in a straight line.
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Have a look when next doing 50mph (speedometer) in a 50 zone on the motorway how many trucks with accurately calibrated speedos come past you...
Not neccessarily the case that in this instance the truck is actually doing 50mph. Couple of days ago i was driving along a single carriageway road doing an indicated 50mph (the legal limit for a van/LCV) and i was overtaken by an articulated truck who, legally, should have been doing 40mph. Fortunately the police around here take the sensible view that an HGV doing 40mph on these kind of roads is likely to cause more accidents through impatient drivers perfroming dangerous overtaking manoeuvres and so don't bother the HGV drivers too much unless there happens to be a current 'anti-speeding campaign'.
As to the GPS vs speedo accuracy, i'd also tend to think the GPS would be more accurate, though maybe not 100%.
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Not neccessarily the case that in this instance the truck is actually doing 50mph. Couple of days ago i was driving along a single carriageway road doing an indicated 50mph (the legal limit for a van/LCV) and i was overtaken by an articulated truck who, legally, should have been doing 40mph. Fortunately the police around here take the sensible view that an HGV doing 40mph on these kind of roads is likely to cause more accidents through impatient drivers perfroming dangerous overtaking manoeuvres and so don't bother the HGV drivers too much unless there happens to be a current 'anti-speeding campaign'.
Unless you're in Scotland, the limit for HGVs on single carriageway roads is now 50mph. It changed about 3-4 years ago I think. Has made a huge difference in the wide open rural roads of the north of England where it's safe for an HGV to do that speed, probably less so in areas like Cornwall with winding roads and the lorries are still doing 30.
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Raising the speed limit for HGVs from 40 to 50 mph has made little noticeable difference since few lorry drivers kept to 40 anyway.
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There are regular threads started on here about this. AFAIK, manufactured speedos are not permitted to under-read, but may over-read by up to 10% to ensure a 'safety margin' for the driver. You don't need to check with GPS (which can be approximate), just look at the automatic signs at the start of some speed zones. I think those are pretty accurate and usually show less than your speedo.
Convictions are based on actual speeds plus a bit over, not on what your speedo indicates.
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On a fairly straight flat road I think GPS is as accurate as it can get without some very sophisticated equipment.
GPS is apparently accurate to around 3m (10ft). This is consistent with what I find with the Garmin when approaching a junction.
So on a flat stright bit of road 300m long, the GPS is probably accurate to +/- 1%. This represents 0.5 mph at 50 mph.
For the purposes of speedo calibration GPS is probably more than accurate enough - on an analogue mph display 0.5mph would repesent approx 1 degree change in the needle.
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45 when doing 40 is slightly higher than I have experienced, but not OTT.
There is another test you can do.
I have found rev counters far more accurate than speedos.
In top gear both our cars do 25 mph per 1,000 revs. At 2,000 rpm the speedo reads about 53-54 mph, GPS readout is exactly 50mph. At 3,000 revs, speedo close to 80, actual speed 75 mph.
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The rev counter is still, however, subject to variables - i.e. tyre pressure and tread depth. Someone with a greater recollection of pi could calculate the difference between the rolling circumference of a tyre with 7mm of tread and one with 1.6mm. GPS removed the dependency upon that variable.
Edited by mcb100 on 31/07/2019 at 14:19
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Now, my Google maps app has its own speedometer and I've noticed that when my car says I'm doing about 45, GM displays 40 roughly. Is it possible that GM is wrong, or is it definitely the car?
Incidentally, I’m a full time Google Maps user on iPhone, and have yet to see the speed display on screen (it’s there on Waze) - I’m wondering if it’s just an Android feature?
Use Google maps and Waze on Android Auto on both of our cars (cars are 2017 and 2018, phones both Android 9) thus up to date.
Maps only shows the speed limit, Waze shows limit and actual speed (if you set it in the menu).
I suspect the OP thinks that the displayed 40 is his speed but in truth it the limit.
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A quick look on Google does show a gradual roll out of the speedo to Google Maps, but I haven’t seen it yet. Maybe on the next app update.
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I use Google maps on Android all the time. It doesn't display limits, it does as of recently display speed.
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YAWN should be concentrating on the road!
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Even I'm not that stupid. 40 was just an example. I've noticed the speed disparity from 10-70 MPH. It's not displaying the limit, it's displaying my speed. I use Android too and the speed display only appeared recently
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Maps only shows the speed limit
Went into Barnsley this morning, plugged in phone and started up Anroid Auto to see if Maps does have a speedo.
It doesn't.
And today there were speed limits shown.
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Round trip to Scotland recently of 1200 miles. I checked my Golf speedo against the digital roadside panels that you see in 30mph limits. Checked speedo 11 times and it was consistently indicating 2mph faster than the panels.
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Unless you're in Scotland, the limit for HGVs on single carriageway roads is now 50mph. It changed about 3-4 years ago I think. Has made a huge difference in the wide open rural roads of the north of England where it's safe for an HGV to do that speed, probably less so in areas like Cornwall with winding roads and the lorries are still doing 30.
I wouldn't swear to this, but i think the 50mph limit for HGV's is just on the A9 from Perth to Inverness. I remember hearing about it being discussed on radio Scotland about the same time average speed cameras were being rolled out for the whole stretch. The reason being that as it is such a busy road, having trucks sitting at 40mph was likely to cause accidents as frustrated drivers make reckless overtakes.
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Unless you're in Scotland, the limit for HGVs on single carriageway roads is now 50mph. It changed about 3-4 years ago I think. Has made a huge difference in the wide open rural roads of the north of England where it's safe for an HGV to do that speed, probably less so in areas like Cornwall with winding roads and the lorries are still doing 30.
I wouldn't swear to this, but i think the 50mph limit for HGV's is just on the A9 from Perth to Inverness. I remember hearing about it being discussed on radio Scotland about the same time average speed cameras were being rolled out for the whole stretch. The reason being that as it is such a busy road, having trucks sitting at 40mph was likely to cause accidents as frustrated drivers make reckless overtakes.
I think the point being made was that the HGV limit on NSL single carriageways is 50 throughout England & Wales - in Scotland it's still 40 apart from the A9 Perth-Inverness as noted above.
In reality few truckers stuck to 40 on the A9 before it changed, some of them riding their limiter at 56.
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Went into Barnsley this morning,
Ah ha, always guessed as much! What do they say, You can always tell a Yorkshireman...? :-)
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Google maps ,on my Android shows the speed limit only.
But Waze, owned by Google iirc, shows the speed limits plus a speedometer.
Edited by Glaikit Wee Scunner {P} on 01/08/2019 at 21:50
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When I got pulled over by French police some years ago for driving at 150kph my speedo agreed with the radar device they showed me, no leeway there, speedo was accurate. I was surprised with the accuracy, but too late by then.
I've never been able to test since, my dashcam does not have GPS.
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Using Waze versus both our Suzuki cars Cruise control. I've set it at 71 in a 70 zone and the GPS consistently shows 68mph. So under but not too bad.
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