OT: Digital Speedometer

[Harry Bloomfield, Esq.]

Fred formulated on Tuesday :

If you know your speed accurately, why would you want to do that?

[Roger Mills]

My car has an analogue speedo, and can also display speed digitally. They are both derived from the same source, and always agree with each other - which means that they're both anything up to 10% fast. I need to use my stand-alone GPS to get an accurate speed reading for setting the cruise control.

[Brian Gaff (Sofa]

For Digital Speedos you need to only measure digital miles an hour which supposes there is such a thing as a digital mile. There were certainly digital nautical miles as we used to have a Decca navigation device when I worked that said it measured them. Brian

[Fred]

We'll see...

Irrelevant to what others choose to do in traffic jams etc.

I do just that because I now hardly ever drive an auto anymore, just a manual.

[Fred]

I don't believe that with changes in tyre pressure.

[Fred]

Some of us choose to drive at the speed that we know will not see us get booked.

[Harry Bloomfield, Esq.]

Brian Gaff (Sofa) explained :

Modern speedos tap into one of the ABS sensor at the wheel, to measure wheel rotation speed and thus road speed. The only error is the charging diameter of the tyre, as it wears.

Earlier systems would monitor the output shaft of the gearbox, either digitally or as a mechanical drive.

[Fred]

So your digital speedo can't actually be 100% accurate.

[NY]

I occasionally used left-foot braking to give me extra control of my speed and position when manoevring in a tight space in an automatic, but I would never trust myself to use it at greater than walking pace. My left leg has developed 40 years' muscle memory of being used for large movements of the clutch, unlike my right leg for fine movement of accelerator and brake. If I used my left leg to brake, there is too much temptation instinctively (eg in an emergency, when action comes *before* thought) to make a clutch-sized movement and lock the wheels.

On the few occasions I've driven an automatic, I tried to drive it as much like a manual as possible: right foot for braking except at very low speed and always put the car into neutral and apply the handbrake when stopped in traffic. I was trained up by an instructor who drummed into me "footbrake for stopping; handbrake for *staying* stopped" and I really hate people who sit in a queue of traffic with the brake lights dazzling me (*). The only time I keep my foot on the footbrake (as well as putting on the handbrake, instinctively!) is when I'm stopped in the middle of the road waiting to turn right into a side road, and I want to make it blatantly obvious that unlike all the other cars on the road, I'm stationary. My sister was once rear-ended by a driver in the US who blamed her for not keeping her brake lights on and said that the convention over there was only to come off the brake as a signal that you were setting off.

(*) One night I got caught up in a queue of mostly stationary traffic for half an hour as it very gradually inched forwards to get round an accident. (**) It was dark so my eyes needed as much night vision as possible. The guy in front kept his brake lights on all the time, apart from the brief seconds while he moved forwards, and after a few minutes of this I found it very difficult to see the road ahead when it was my turn to move. I resisted the temptation to put my headlights onto full beam to dazzle him (via his mirror) as he was dazzling me, but the guy behind me who was also dazzled by the same car, was not so restrained. After a further few minutes when this had no effect, he got out and "had words" with the driver and told him to do everyone behind me a favour by using his ****ing handbrake.

(**) It was the classic case of the main road which had been designed to take a large volume of traffic being blocked by a head-on smash, so all the traffic was diverted onto a road that couldn't hope, with traffic lights that only let a few cars through in any direction. I tried to remember the map of the area to see if there was a better way than taking the way that the police were diverting everyone, and decided that the River Thames only had a few bridges round there, so I'd have to take a *very* long diversion to get round this one pinch point. I was late home that night, and it was before the days of mobile phones so I couldn't let SWMBO know what had happened.

[Vir Campestris]

Let me be different...

My analogue speedo reads high by about 8% across the range.

OTOH my tacho shows that top gear is 20MPH per thousand RPM. Bang on, all across the range. So I usually set my cruise speed on the tacho - the limits are all when the needle is on one of the lines.

All calibrated from the standalone GPS.

Andy

[NY]

I think you mean over-read (ie they display 60 when you're doing maybe 55).

Maybe what he meant was "all speedos that measure rotations of the wheel over-read". This is because it is an offence for a speedo to under read, since it would mean you were speeding even though your speedo said you were not. Since all mechanical devices (analogue gauges) have a slight tolerance which may vary with temperature and age of the spring in the dial, they set the systematic over-reading to be greater than the expected random variation - at 60, a gauge set at 65 +/- 3 mph will read 62 - 68 but never less than 60 so you will always be going *slower* than the speed limit if you set the needle at that number. Even if the speed is measured digitally by sensing and timing the interval between one rotation of the wheel and the next with a Hall Effect sensor, the weakest link will be the spring in the analogue gauge which displays that speed.

My car (as compared with a GPS speedo app on my phone) seems to read 5 mph fast for all speeds between 20 and 80, so I am sometimes tempted to drive at

5 mph faster than the limit, as judged by the analogue speedo.

[NY]

I had just got off the Isle of Wight ferry and the ramp was a bit bumpy. That may have affected the filaments in my headlights. I was driving across the New Forest, many miles of unlit, unfenced road with the added hazard of ponies that wander across the road, totally fearless of traffic. Hence the blanket 40 mph speed limit. And suddenly one of the headlights went out. OK, I could still see fairly well by the other one, so I decided to carry on until I got to a town where I could change the bulb under streetlights. Then the other headlight went out. I was now driving blind - at that moment there were no other cars ahead or oncoming to judge where I was in relation to the kerb or to check for ponies. It took me several seconds to remember that I could use my full beam. With hindsight I should have used the front foglights because those might have dazzled oncoming traffic (which by now was plentiful, just when I didn't want it) a bit less. So I had to pull off onto a safe bit of verge, avoiding all the ditches, and change the bulb by feel because I needed both hands and didn't have a spare one to hold the torch. At least I had *one* headlight for the rest of the journey home. It was late at night so no garages were open to buy a *second* spare. Even the motorway service station didn't have any. They had loads of 24 V ones for trucks, but no 12 V ones for cars. You tend to expect that a service station would be sure to sell simple spares like that.

Moral of the story. Don't buy cheap bulbs. I was always having problems with headlight bulbs blowing - about every few months. I always replaced them with cheap ones from a motoring shop (which I won't name, but it's a big name in the UK). Then my local garage suggested trying more expensive branded bulbs. And I don't think I've had a blown bulb since then. The garage confirmed that my voltage regulator was working fine, so it wasn't over-voltage from the alternator - just a combination of bloody atrocious road surfaces and cheap low-quality bulbs.

[JohnP]

Mine uses its camera to read the signs.

[Dave Liquorice]

The analogue displays in my car are driven from stepper motors. On power up they do a self test of going full scale and back.

The analogue and digital displays agree with each other and over read. True 30 mph from GPS or roadside "Your speed is ..." signs displays as 32. True 60 is neared 64. When setting cruise I compensate accordingly.

[Graham.]

IRTA double declutch!

[Dave Plowman (News]

Yup. I'm told Honda do a version which also has a torque converter just for moving off. But not heard how it works in practice.

Very low speed shunting - like parking in a tight spot - is easier with a TC transmission. Other than that, the PDK etc as near as dammit perfect.

[Fred]

Yep. missed that.

It was more because the accuracy varies with the tyre wear and tyre pressure. They don't want to be accused of misleading the driver by showing slower than the real speed.

It isnt random.

[Dave Plowman (News]

Yes- far better to carry around a vast weight of batteries at all times. Oh - you're EV would be a very different vehicle without the same degree of complexity as a modern IC vehicle. Think a milk float.

[The Natural Philosopher]

mine is consistently 7% above

All my speedos ever have over read. Usually by around 10%

[The Natural Philosopher]

My digital display (cruse control) over reads by the same amount as the dial

All of the ones here are spot on with my satnav