No Tailgating!

[wolfie]

I have a feeling that my analogue speedo might be slow, though? I was using Sean the Satnav to help me through the tough bits (A43, I'mma lookin;' at you!) and he regularly reported a speed 5mph less than what my speedo was saying - if it said 70, he said 65. Which should I trust?

I'd trust the satnav. It calculates your speed by change in position/change in time, with both the position and time derived from the satellites being extremely accurate. Altitude reports on a GPS aren't quite as accurate, but shouldn't be too much of a concern when you're in your car.

Your car's speedometer is subject to a lot of variables. As the tires wear, it takes more turns per mile, but the speedometer doesn't correct for that. Also, if your tires are underinflated, it will take more turns per mile. Some time back (this was partially responsible for phasing out purely mechanical speedometers), many countries passed laws requiring that a speedometer NEVER show a speed less than the actual speed (presumably to eliminate an excuse by people caught speeding). A result of this is that car makers designed their speedometers to be slightly "optimistic" even in a worst-case scenario (brand-new tires, if there are multiple sizes approved for the car, use the one with the largest tread diameter, and somewhat overinflated), meaning they'll be even more "optimistic" under normal circumstances.

[Chromatix]

It is very common for speedometers to be calibrated to "over-read" slightly, for the reasons that wolfie mentioned. Police cars have their speedos calibrated very frequently to account for tyre wear and so on, so that they can be used reliably to collect evidence about speeding offences.

Trains are generally calibrated so that their speedometers are accurate on brand-new tyres (yes, train wheels have tyres too - they're just made of steel instead of rubber, and are not inflated). As the tyres wear down, the speed starts to over-read, which is a "right-side failure" in industry parlance. The error is usually only a couple of mph, which has a negligible effect on timekeeping for any sane schedule. On railways where timekeeping is considered especially important (eg. Japan), the speedometer is backed up by the driver comparing passing times between his watch and the timetable.

Digital speedometers are subject to the same variation in reading due to changing tyre diameter. This is because nearly all speedometers are based on the same principle of measuring axle rotation. The only difference is that a digital speedometer, provided it is based on a digital rotation sensor, is easier to calibrate and does not "drift".

[PepperElf]

hmm that may explain my own car then. on a whim once we used a handheld gps to check speed. at about 70mph it was reading a few mph slower.

although on those 'your speed is' signs, when i'm doing 25 it seems to read a bit closer.

i just figured that it was cos of the year my car came out in - 01. back then limits were a bit lower in most spots, so it may have been calibrated for higher accuracy at 65 and lower, and when i'm going 70 on the highway here it's outside my optimum range for speedometer accuracy.

or so that's my theory at least. haven't really tried proving it yet tho.

On railways where timekeeping is considered especially important (eg. Japan)

You can set your clock by their trains. It's that serious in Japan. If the train actually runs late (rare) they issue you a ticket to bring to work. IIRC most companies there will accept this as proof.

[Caractacus_Potts]

There was a story in the news a little while ago about a woman in the fast lane of a highway during rush hour being pulled over and ticketed by a state trooper for going 1 MPH under the posted maximum speed limit. I get a bit frustrated with people who drive below the speed limit but I think that was a bit extreme and I hope the woman went to court and got the ticket dismissed (I think the officer ticketed her causing a road hazard).

Nowadays, in my ripe old age, I tend to drive about 5 MPH over the posted limit. When I come up behind someone going slower it doesn't really bother me if they are going the speed limit. As mentioned above I do get frustrated if they are going below the speed limit but in either case I don't tailgate because it's better to be frustrated than to be frustrated and found at cause for rear-ending the person in front of you when they brake suddenly.

[ADeMartino]

As a general rule, I keep my speed to 55-60 miles per hour, regardless of whether the actual limit is 55 or 65. Why? My little truck gets its very best gas mileage in that range. However, since I AM driving slower than most other traffic, I usually keep my snail's pace confined to the right-hand lane, where it belongs. Doing so allows faster traffic to pass me on the left - which is the way it's supposed to be. Driving slow in the left (fast) lane means overtaking traffic has to pass on the right, which is more dangerous for EVERYBODY involved - and I think that's why the woman you mentioned got a ticket. Not necessarily for driving 'slow' - but for doing so in the inappropriate lane.

It's a chronic problem around here - there always seems to be someone who can't grasp the whole 'slower traffic keep right' concept, and it invariably puts a gigantic knot in otherwise free-flowing traffic.

But, the reverse is true, too - there's always somebody who thinks it's perfectly okay to go 80 in the right-hand lane.

[Geek King]

There was a story in the news a little while ago about a woman in the fast lane of a highway during rush hour being pulled over and ticketed by a state trooper for going 1 MPH under the posted maximum speed limit. I get a bit frustrated with people who drive below the speed limit but I think that was a bit extreme and I hope the woman went to court and got the ticket dismissed (I think the officer ticketed her causing a road hazard).

Actually, some places in California (and maybe others) will do this, since studies of heavy traffic flow have shown that people doing 5-10 mph below the rate of travel of other traffic can create traffic jams behind them. You can indeed get a ticket in certain situations for -not- speeding if the rest of the traffic is.