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Thread ID: 135704	2013-12-03 01:03:00	In light of the 4kmh tolerance for speeding...	tingle (6539)	PC World Chat

Post ID	Timestamp	Content	User
1361513	2013-12-03 01:03:00	Roadside speed monitors usually show me at 4 -5 kmh lower than my speedometer. The tomtom GPS unit reports me at 2 - 4 kmh lower than my speedometer. Should I average them out and hope for the best? :groan: How do I know what to believe? Which should I trust? www.stuff.co.nz	tingle (6539)
1361514	2013-12-03 01:35:00	Is that at 50k ?? at 100k the difference may well be greater. Problem is that we dont know the margin of error on GPS & Roadside speed monitors. Changed you car tyres in the last 10 years?? That may very well have a big impact on speedo errors Solution : driver everywhere at 40K (& 90k) & be well withing the limit . Who comes up with these rules? 55km/h & 105km/h ARE NOT unsafe speeds . If these speeds were as bad as they seem to be implying, we all would be crashing every other week.	1101 (13337)
1361515	2013-12-03 01:46:00	55km/h & 105km/h ARE NOT unsafe speeds ... sometimes. 55km/h & 105km/h are unsafe speeds... sometimes.	coldot (6847)
1361516	2013-12-03 02:05:00	if 50k & 100K are 'safe' , I can bet that 55k & 105k will still be safe . Its the driver, not the speed thats unsafe. We use these same roads for car racing events, driving at much much higher speeds . Rallying is on our worst roads at mindbogglingly stupid speeds. Yes if you do stupid things, in theory no speed will be safe . So we set the speed limit at zero ;) There is allways some risk in everything we do. 4k tolerance will achieve what ??	1101 (13337)
1361517	2013-12-03 02:22:00	Its not the speed that kills, its the sudden stop resulting from the impact sustained when you meet that car coming around the bend on the wrong side of the road. If it was speed that killed us then every F1, Rally and Indy Car driver would be killed once they went over 100kph on the circuit... hmm not so much. I would agree though, there's no point trying to pass that slow caravan hogging the road in front of you because you are just going to get stuck behind the slow Asian driver that's next in line. ;) So just sit back, cruise and 90-100 kph and enjoy the slow moving scenery..... Or.. start your journey early in the morning or don;t travel on Boxing Day cause then you are asking for trouble :)	Webdevguy (17166)
1361518	2013-12-03 02:38:00	Yeah sorry, that is at 50kmh speeds. If I travel at 40 k or 90 k, I would have so much road rage behind me!	tingle (6539)
1361519	2013-12-03 03:08:00	Cruising on the northern motorway, I closed in on a police car going slightly slower. I nailed the speedo at 100 kph, and drew abreast of the officer who looked over at at me and started to shake his head. I got the hint and slacked off. Later I checked the speedo against the GPS to find 100 on the GPS = 97 on the speedo. 100 on the speedo = 104 on the GPS. Thanks officer for the heads up, I'll rely more on the GPS in future and keep it turned on.	WalOne (4202)
1361520	2013-12-03 03:13:00	Its not the speed that kills, its the sudden stop resulting from the impact sustained when you meet that car coming around the bend on the wrong side of the road.Indeed. To quote Jeremy Clarkson: Speed never killed anyone. Suddenly becoming stationary, that's what gets you.	pcuser42 (130)
1361521	2013-12-03 05:22:00	If you want to use your GPS to test your speedometer find a long flat straight section of road and drive at a constant speed, preferably in a 100km/h zone. GPS is most accurate in this situation as it can't respond to sudden changes in speed and doesn't account for hills or corners. It's actually really accurate in this situation as all it's doing is measuring the time it takes you to travel between two points and calculating the speed based on that. Cruise control helps. If your speedo is reading low you should get it calibrated. I was initially a bit annoyed thinking that the accuracy of a speedo isn't good enough to ensure I won't speed by accident, but then I remembered mine always reads a little higher than my actual speed and did a bit of googling. Turns out a speedo reading low is very unusual and against the law and that most manufacturers deliberately set them to read slightly high to ensure this doesn't happen. The upshot is if you stick to 100 km/h on your speedo the chances of you actually speeding due to inaccuracy are almost Zero. I still don't like it though, it's a very small margin for error. If the road suddenly dips downhill or something gaining 5km/h briefly unintentionally is incredibly easy and breaking suddenly on a straight road in this situation could be more dangerous than the "excess" speed. All these speed cameras and low tolerance campaigns seem to be working though, if I set my cruise control at 100km/h on the highway I find myself catching the traffic more often than not and sometimes passing it comfortably. It's even more apparent if I set it at 104 which I know is actually 100 for my car based on my phones GPS and those roadside trailers with the speed readout (which agree exactly with each other). On a side note, I read an interesting comment on one site. It's repeated everywhere that changing your tyre pressure alters the speedometer accuracy. One guy disputed that, reasoning that the same amount of rubber has to travel along the road for each revolution whatever the tyre pressure is and I can't fault that logic other than perhaps if the rubber stretches. What do we think of that one?	dugimodo (138)
1361522	2013-12-03 05:50:00	On a side note, I read an interesting comment on one site. It's repeated everywhere that changing your tyre pressure alters the speedometer accuracy. One guy disputed that, reasoning that the same amount of rubber has to travel along the road for each revolution whatever the tyre pressure is and I can't fault that logic other than perhaps if the rubber stretches. What do we think of that one? changing tire pressure changes the radius of the tire (note: i say radius not diameter). a flatter tire is slighter closer to the road so you will go slightly slower for the same speed reading. however the catch is that the difference is small. most car tires the difference is barely measurable until tire is so deflated it will destroy itself if you drive on it. you can change to a different sized tire and the speed change is only 1%.	tweak'e (69)
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