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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 | ||
| 1361523 | 2013-12-03 06:32:00 | 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. Exactamundo!!! I have done the same test with both with a Garmin cycle computer an Android app called Android Speedometer (play.google.com) - I got the same results as you did. So I just set the cruise control to about 106 (and then go back to the back seat and make a cup of tea :D), and I never have a problem. In fact, I sometime prop up my Nexus 7 in front of the speedo and use that instead... Dugimodo is correct - speedos will always read higher than the speed you're actually travelling, so anyone who still gets a ticket is an idiot |
johcar (6283) | ||
| 1361524 | 2013-12-03 07:16:00 | Most speedometers will read high. They are allowed to. Depending on the car, conditions, etc, it can be anything from about 1% - 10%. Most manufacturers adhere to the common international requirement (most of europe etc) that the speedo cannot read low from the factory, so they go the other way and make it read high for safety. I believe the average is about 3% variation, so speedo = 100, means 97kmh real speed. A quality GPS is more accurate for sustained speed, but take longer to adjust if you are accelerating and decelerating. Tyre size and pressure can certainly impact it, but unless these are varying greatly, the change should be minimal. Having your tyres a few psi low will not be noticable, but near-flat tyres vs fully inflated would be, due to the change in rolling diameter. Likewise, ify ou've put different sized wheels or tyres on the car than what came from the factory, you should really get your speedo recalibrated. | inphinity (7274) | ||
| 1361525 | 2013-12-03 16:48:00 | Thanks for all the comments and advice. I will try your suggestion dugimodo. If I still get a ticket, will you guys all pitch in and help me pay for it?? :D |
tingle (6539) | ||
| 1361526 | 2013-12-03 18:42:00 | if 50k & 100K are 'safe' , I can bet that 55k & 105k will still be safe . Its the driver, not the speed thats unsafe . There is no one safe speed . It depends on the road, the weather, the condition of the road, traffic, time of day, all sorts of things . The driver - yeah, the ones who think they are brilliant drivers, they can control a situation if it arises unexpectedly . The faster you go, the harder it is to stop in a safe distance . . . . . . they laws have to have some speed limit, they can't just randomly suggest something - but they do have those recommended signs on some roads, so trying to get it through that it is up to you to adjust speed accordingly and not just hold it at 1km either side of the limit come what may . Most drivers are worse than they think they are anyway . |
pctek (84) | ||
| 1361527 | 2013-12-03 19:05:00 | 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%. Ah yes I know and that's the basis everyone uses for saying it alters the speedo reading and it would be my first thought also. The point is though, as you mention, the diameter and therefore the length of rubber that travels along the road for every revolution is the same and therefore the distance travelled is also the same, isn't it? the tyre presumably simply becomes less round with lower pressure but that doesn't make it any shorter around the outside. I'm going to google it :) |
dugimodo (138) | ||
| 1361528 | 2013-12-03 20:00:00 | ^ The circumference of the undeflected tire is constant, but the travel distance is different to the effective circumference of the underinflated tire. The difference is made up by the tire tread area being compressed before the road contact point and resuming it's relaxed length and thickness after it lifts from the road contact area. This takes a considerable amount of energy, which appears as heat in the tire and disappears as excess fuel used from the tank. | R2x1 (4628) | ||
| 1361529 | 2013-12-03 20:03:00 | I think youll find most manufactures build in a -10% error into speedos so that the Americans don't sue them for their traffic fines :D | beama (111) | ||
| 1361530 | 2013-12-03 21:33:00 | ^ The circumference of the undeflected tire is constant... etc. OK makes sense. And by Diameter above I should have said circumference as you did. Anyway my research on google indicates that while you are right and there is a difference real world tests (by some mechanic on the myth busters forums for example) show that it's so tiny as to be un noticeable. The Steel belt prevents significant compression or strectching under normal conditions. Basically Tyre pressure does not effect speedometer readings in any significant way. The biggest culprit apparently is people changing wheel types or tyre profiles which does have a noticeable effect on speedometer readings, you should stick to manufacturer reccomended tyre sizes unless you plan to get your speedo re calibrated. |
dugimodo (138) | ||
| 1361531 | 2013-12-03 21:53:00 | My Navman reads a few Kms faster than my car's speedo ... Ah .. just stay home during the silly season. Coffee, milk, bread, ham maybe some cheese and head to Pak N' Sav. A bargain. I looked for some scotch steak but compared to what you guys get at some of the smaller marts in AKL ie $25/kg, our local Countdown was $30. Ah .. pass :D |
Nomad (952) | ||
| 1361532 | 2013-12-03 22:14:00 | ^ The circumference of the undeflected tire is constant, but the travel distance is different to the effective circumference of the underinflated tire. The difference is made up by the tire tread area being compressed before the road contact point and resuming it's relaxed length and thickness after it lifts from the road contact area. This takes a considerable amount of energy, which appears as heat in the tire and disappears as excess fuel used from the tank. do you also write the fine print on legal documents :lol: |
beama (111) | ||
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