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Speed v Distance
Comments
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I had one vehicle where after me covering tens of thousands of miles there was a discrepancy of tens of miles between one of the trip computers which had never been reset after I had reset it when picking up the car and the odometer.0
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As far as I'm aware those speed indicator signs on the side of the road are not calibrated to any real accuracy so cannot be used to calculate true speed.0
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Comparing against GPS speed, I have found that my speedo is a consistent 3MPH high between 30 & 70 MPH compared to the GPS speed.0
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You can check the odometer against the driver location signs on a motorway.Take a passenger who has good eyesight and a pen and paper.The signs are 500m apart, except when they aren't
, but the bottom number gives the distance in km from some unknown datum. (Top is the road, middle is direction of travel, bottom is distance from datum)
So, with some difficulty it should be possible for the passenger to note the distance on the first sign as you pass it and zero the tripmeter, then 9miles is 14.5km to around 0.1%, so they need to count the signs to the 29th (the start is zero), or read the distances off the signs until they get to 14.5km from the first sign and you read the trip distance.I want to go back to The Olden Days, when every single thing that I can think of was better.....
(except air quality and Medical Science)
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In sailing, the coast is peppered with (nautical) mile markers and I'm always amused that on the chart it has a line between them stating 1852m instead of the more natural 1nm.
https://www.friendsofstokesbay.co.uk/measured_mile_markers/
Could you use an app like strava to measure distance travelled as you will get a more accurate measurement the further you travel since timing a 500m mark would be more prone to error than comparing a 5 mile run.
On the topic of time/distance, many people aren't aware that the tachymeter dial on your sporty watch is a calculator exactly for this purpose. In the example below, if you covered a measured mile and the second hand had timed 15 seconds, your speed would be 240mph (ie 4 miles a minute). In a full minute, it's 60mph. 51.4 seconds = 70mph etc.
Although I appreciate most people don't wear watches anymore ;-)
Signature on holiday for two weeks0 -
facade said:You can check the odometer against the driver location signs on a motorway.Take a passenger who has good eyesight and a pen and paper.The signs are 500m apart, except when they aren't
, but the bottom number gives the distance in km from some unknown datum. (Top is the road, middle is direction of travel, bottom is distance from datum)
So, with some difficulty it should be possible for the passenger to note the distance on the first sign as you pass it and zero the tripmeter, then 9miles is 14.5km to around 0.1%, so they need to count the signs to the 29th (the start is zero), or read the distances off the signs until they get to 14.5km from the first sign and you read the trip distance.
We are all in the same boat anyway0 -
On a modern car the speed is picked up by the wheel speed sensor, one of the same sensors that the ABS uses.
Years ago when dials were analogue they were driven by a small gear on the side of the gearbox.
Both these signals would also drive the mileage counter, so they would be "off" by roughly the same amount.
Modern cars work it out all electronically, older cars with analogue dials used a load of cogs and gears.
Speedos cannot under read, they cannot tell you you are going slower than you actually are.
It's been a while since I looked, but there is/was an actually figure that they were allowed to over read at, something like 10% plus 6mph.
As cars have become more electronic the speedos and mileage counters have become more reliable, but they are still not allowed to under read.
It's become far easier to program the car with the correct wheel/tyre size, where in the past it was an expensive job to calibrate an analogue speedo for the same model cars that were available with different size wheels/tyres.
I would guess your test with the road side speed check cameras is probably down to a bit of both.
Yes, your speedo is over reading.
Yes, the road side speed check camera is over reading a little too, after all they want you to slow down or at least keep a closer eye on your speed.
I would say that unless you have swapped wheels and tyre sizes, your speed and mileage would be within 10%.
It is a bad thing for you and your car?
Not really, every car is roughly the same.
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Your spedo is reading from your ECU, and its calculated electronically from your gearbox.Speedos are calibrated from factory on a set of new tyres, so from factory with a new set of tyres, the speed of the car is going to be pretty accurate.Once those tyres have done 30K miles for example, its rollign diameter will have gotten smaller by approx 12mm, this will mean that the spedo will over read (you will be going slower than it the spedo says)in reality, a new vs old trye, is travelling approx 4mm less for every revolution.0
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sk2402005 said:Your spedo is reading from your ECU, and its calculated electronically from your gearbox.Speedos are calibrated from factory on a set of new tyres, so from factory with a new set of tyres, the speed of the car is going to be pretty accurate.Once those tyres have done 30K miles for example, its rollign diameter will have gotten smaller by approx 12mm, this will mean that the spedo will over read (you will be going slower than it the spedo says)in reality, a new vs old trye, is travelling approx 4mm less for every revolution.0
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Goudy said:
It's become far easier to program the car with the correct wheel/tyre size, where in the past it was an expensive job to calibrate an analogue speedo for the same model cars that were available with different size wheels/tyres.
If it were any different, then cars might look daft across a model range having huge gaps in the wheel arches on the 1.1L model whilst the 3.0GT looked more buxom.
https://www.willtheyfit.com/
Signature on holiday for two weeks0
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