How exactly does a car's/boat's/plane's speedometer work?

I always wondered about boats (let's say small pleasure boats), but thinking about it, it isn't so obvious with cars either.

For example changing the tire's size shouldn't effect the measured speed.

How about planes? I guess nowadays with GPS it is easier, but how did they measured the speed earlier and how did they adjust for wind?

 

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Planes had (and still have) a device called a pitot tube.
This has two openings: one directly in front (that checks air pressure from the movement) and another in the side of the tube (that checks ambient air pressure, except for minor boundary layer differences).
Then it's a simple matter of comparison between ambient (i.e what the air pressure is stood still - and takes into account any wind) and incoming flow.
The difference between the two gives the speed.

http://en.wikipedia.org/wiki/Pitot_tube

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Well, here is the answer to the boat.

http://www.ehow.com/how-does_4739856_boat-speedometer-work.html

Whatever I have found for cars and planes didn't adjust for tiresize and wind...

 

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Sure but how do they know what is wind and what is the airplane's relative speed? When flying across the ocean they told us the traveling speed and also the wind's direction and speed.

How did they measure windspeed 20-30 years ago on an airplane??

For boats, with a simple trick, you can measure the speed of a river or current. You just go with the speed so you are not moving compared to the shore and that gives you the river's speed, thus you can calculate your traveling speed if straight up or down, but if the river is wide enough and you are criss-crossing, you can't calculate your traveling speed compared to the shore...(only approximately)

 

Car tyre size can't alter too much because then the tyres wouldn't fit inside the wheel arches.
In the UK (at least) there's an "allowed" discrepancy of +/- 10% in a speedometer reading, more than enough to account for tyre size variation.

Wind is accounted for in planes by the side holes in the pitot - if there's any wind it becomes part of the "static" reading.

 

That's what the side-opening holes do: they take a measure of the air pressure as it would if the plane were not moving. It senses the pressure variation for altitude and any wind.
The front opening gets the pressure from wind, the plane's movement and altitude.
Subtract side reading (wind + altitude) from front reading (wind + altitude + movement) and the result is movement.
And the atmosphere is pretty well charted for altitude variation - at height X the pressure will drop by so and so, so that the side hole value can be compared to the known typical altitude pressure to get wind speed.
Direction can be found by several methods, one of them being how much "rudder" needs to be used (i.e. which way the aircraft has to be pointed) to maintain a set heading.
If the wind's from the left the aircraft has to fly slightly rightwards to actually go straight.

 

A 10% smaller/bigger tire in diamtere should cause the same 10% missreadings in speed. Certain cars can go with way bigger than normal tires.

In high altitude flying winds can be a substantional component and can add or subtract of the aircraft's speed seriously.

 

Yup, but I'd bet that they have change the speedo.
That's why car manufacturers generally list acceptable tyres, and sizes. There won't be anything like 10% variation in tyre diameter for "approved types", or between brand new "approved large size" and well-worn "approved small size".
The average tyre is what, 400-450 mm diameter, wear won't account for 10% of that, you'd be through the rubber completely.

Hell yeah, the jet stream can run up to around 200 mph (max) IIRC.

 

I bet this is more than 10% bigger in diameter than the original tire:

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But the whole point is that it is possible to change your tiresize and that would mess up the speedometer...

 

Not entirely true James, reasonable estimates could be made even before GPS etc, otherwise transatlantic flights wouldn't have been a proposition before that.
IIRC even Alcock and Brown managed to land somewhere near their intended landing zone, and that was pre WWII.
They had a sextant and took star sightings (that I know of - dunno exactly what they did during daylight) and that would give them a current position which compared to their intended position would present an average value for wind speed and direction.
And bomber crews during WWII... (although they had the advantage of being over land mostly - but missions still took place when the ground was obscured by cloud).

 

Agreed, but I did say that I'd put money on there being an adjustment to the speedo to account for the difference.

 

http://www.roversd1.nl/sd1web/wheelcalc.html

http://forums.off-road.com/suzuki-geo-all/150541-fitting-31-tyres-my-jimny.html

http://www.google.co.uk/search?clie...r tyres" speedometer&meta=&btnG=Google Search

 

Seriously, just because I get bigger tires, I wouldn't adjust the speedometer, probably it isn't even possible for the average driver..

About your last quote, who does the calibration??

 

It depends: note the last seven words in the quote I gave in my last post.
You may end up with an illegal vehicle.

 

The speedometer doesn't need to work for the vehicle to be legal. What the quote meant is that with a bigger tire, you undermeasure your real speed, thus you go faster than what you think and you will be speeding>>>>ticket.

 

Two things on that.
One is as you said: you won't know what speed you're actually doing (hence ticket) and the other being that (in the UK at least) the vehicle is not legally road-worthy unless all of the parts are working correctly.
When I said +/-10% that is a LEGAL requirement. If the speedo is off by more than that the entire vehicle is illegal.

 

There are street legal bikes in the US without speedometer. Thus it is obviously NOT a legal requirement. Go figure... (in the US, and who cares about the UK?(where it is required))

If you ever watched Orange County Choppers, they never put a speedometer on the bike...

 

Who can account for foreigners?

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Watched?
I've never even heard of them.

 

The speedometer rule in the US varies by state. I don't think CA requires a speedometer on motorcycles. Some other states may. Cars fall under different rules. I know in Oregon, if you get a ticket, and tell the officer you just had new tires put on recently, he will tell you to go to a shop, and bring the correction paperwork with you and the Judge will drop the charges..... assuming you were not grossly exceeding the speed limit.

Edit: To answer the OP, Tire size affects your speedometer reading. The speedometer cable is generally linked to one of you hubs, and is calibrated based on the recommended tire size for the vehicle. Changing the tire size changes how fast the hub spins for a given speed. If you install exceeding larger or smaller tires, you may be required by law to have your speedometer calibrated.