Speedos and other things

[Theonlysue]

When my merc based electronic Speedo displays 53 mph, my sat nav says I'm doing 50 mph.
Is the total milage displayed on my mh based on what the Speedo is doing, in which case, is it clocking up extra milage unnecessarily???

 

[hilldweller]

I question I've asked myself many times. It would also flatter your mpg. The only way I can think of testing is a long run and compare GPS distance to odometer distance.

The speed +10% is to keep UK law happy, logically miles should be spot on give or take rolling radius errors.

 

[Spriddler]

Ah, THOSE Speedos! I thought you were seeking advice for a friend on budgie-smuggling.
All my vehicles over-read a bit vs the GPS but I'm not bothered that the odometer will read 53,000 when I've covered 50,000 miles.

 

[pappajohn]

The speed shown on the speedometer can never read slower than your true speed by law.
It can show a higher speed within reason... 5 or 10% may be acceptable but its usually nearer to 3 to 5%

The speed shown on the speedometer will have a direct relation to the distance shown on you odometer, there is only one speed sensor so it can't do otherwise
At 60mph after 1 hour the odometer will show 60 miles more even if the speedo is 10% fast.. . The speedo and odo don't know they are fast.

 

[Offagain]

Ah, THOSE Speedos! I thought you were seeking advice for a friend on budgie-smuggling.
All my vehicles over-read a bit vs the GPS but I'm not bothered that the odometer will read 53,000 when I've covered 50,000 miles.

I also thought the subject matter was ill advised male swimming garments, but was intrigued to know what the other things were!

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[klaatu]

As I understand it, electronic speedometers and odometers work by receiving a pulse every time a wheel rotates. It’s thus perfectly possible that the conversion of those pulses to speed is done differently to the conversion to distance. It may not be, I have no idea, I’m just saying that unless you actually know how the internals are programmed, you can’t definitively say that they will show exactly the same discrepancy from reality, even though they are fed from the same sensor.

 

[Deleted member 57092]

The 5% innacurracy or whatever is with new tyres
As the tyres wear the rolling radius decreases so the wheels do more revolutions to cover the same distance so the innacuracy gets worse.
In the olden days when I was an Austin Rover mechanic the calibrated speedometers for police cars had different part numbers dependant on what brand and size of tyres were fitted !
(But they didn’t compensate for tyre wear !)

 

[pappajohn]

As I understand it, electronic speedometers and odometers work by receiving a pulse every time a wheel rotates. It’s thus perfectly possible that the conversion of those pulses to speed is done differently to the conversion to distance. It may not be, I have no idea, I’m just saying that unless you actually know how the internals are programmed, you can’t definitively say that they will show exactly the same discrepancy from reality, even though they are fed from the same sensor.

Then there's only one way to know.. . A measured distance at a constant speed.
Ie: 25 miles @ 50mph = 30 mins dead
Easily do-able using motorway mile markers and a stopwatch.

GPS isn't 100% due to drift.

But to be honest, I don't see how either could be adjusted....both would need a reference point which isn't available.

 

[Burgo]

From Wikipedia :
Excessive speedometer error after manufacture can come from several causes but most commonly is due to nonstandard tire diameter, in which case the error is

{\displaystyle {\mbox{Percentage error}}=100\times (1-{\mbox{new diameter}}/{\mbox{standard diameter}})}

Nearly all tires now have their size shown as "T/A_W" on the side of the tire (See: Tire code), and the tire's

{\displaystyle {\mbox{Diameter in millimetres}}=2\times T\times A/100+W\times 25.4}

{\displaystyle {\mbox{Diameter in inches}}=T\times A/1270+W}

For example, a standard tire is "185/70R14" with diameter = 2*185*(70/100)+(14*25.4) = 614.6 mm (185x70/1270 + 14 = 24.20 in). Another is "195/50R15" with 2*195*(50/100)+(15*25.4) = 576.0 mm (195x50/1270 + 15 = 22.68 in). Replacing the first tire (and wheels) with the second (on 15" = 381 mm wheels), a speedometer reads 100 * (1-(576/614.6)) = 100 * (1 - 22.68/24.20) = 6.28% higher than the actual speed. At an actual speed of 100 km/h (60 mph), the speedometer will indicate 100 x 1.0628 = 106.28 km/h (60 * 1.0628 = 63.77 mph), approximately.

In the case of wear, a new "185/70R14" tyre of 620 mm (24.4 inch) diameter will have ≈8 mm tread depth, at legal limit this reduces to 1.6 mm, the difference being 12.8 mm in diameter or 0.5 inches which is 2% in 620 mm (24.4 inches).

 

[klaatu]

Then there's only one way to know.. . A measured distance at a constant speed.

Yes, absolutely.

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[Deleted member 57092]

We speak the same language
Its just that I use fewer words

 

[Nasher]

Also you need to add to the variable that a tyre will increase in diameter & hence circumference due to centifugal force as speed increases - just to complicate matters

 

[pappajohn]

From Wikipedia :
Excessive speedometer error after manufacture can come from several causes but most commonly is due to nonstandard tire diameter, in which case the error is

{\displaystyle {\mbox{Percentage error}}=100\times (1-{\mbox{new diameter}}/{\mbox{standard diameter}})}

Nearly all tires now have their size shown as "T/A_W" on the side of the tire (See: Tire code), and the tire's

{\displaystyle {\mbox{Diameter in millimetres}}=2\times T\times A/100+W\times 25.4}

{\displaystyle {\mbox{Diameter in inches}}=T\times A/1270+W}

For example, a standard tire is "185/70R14" with diameter = 2*185*(70/100)+(14*25.4) = 614.6 mm (185x70/1270 + 14 = 24.20 in). Another is "195/50R15" with 2*195*(50/100)+(15*25.4) = 576.0 mm (195x50/1270 + 15 = 22.68 in). Replacing the first tire (and wheels) with the second (on 15" = 381 mm wheels), a speedometer reads 100 * (1-(576/614.6)) = 100 * (1 - 22.68/24.20) = 6.28% higher than the actual speed. At an actual speed of 100 km/h (60 mph), the speedometer will indicate 100 x 1.0628 = 106.28 km/h (60 * 1.0628 = 63.77 mph), approximately.

In the case of wear, a new "185/70R14" tyre of 620 mm (24.4 inch) diameter will have ≈8 mm tread depth, at legal limit this reduces to 1.6 mm, the difference being 12.8 mm in diameter or 0.5 inches which is 2% in 620 mm (24.4 inches).

We speak the same language
Its just that I use fewer words

And Wikipedia used a whole library full.

 

[Deleted member 57092]

Good point
Forgot that
And tyre pressure
Atmospheric pressure
Ambient temperature
And wether the tyre has air in it
Or some boy racer has filled it with nitrogen
(Air being 80% nitrogen anyway)

 

[tonyidle]

but its usually nearer to 3 to 5%

You think? A true 65mph read 72mph on the speedo on my last (Hymer) van. The speedo was replaced just before I collected the van (either a recall or faulty) and I suspect that it, and probably the original, were intended for 16" wheels. For some unknown reason Hymer had specified 15" wheels on a Maxi chassis.

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[tonyidle]

Then there's only one way to know.. . A measured distance at a constant speed.
Ie: 25 miles @ 50mph = 30 mins dead
Easily do-able using motorway mile markers and a stopwatch.

GPS isn't 100% due to drift.

But to be honest, I don't see how either could be adjusted....both would need a reference point which isn't available.

I've done both but at a constant speed on a (visibly) level road the GPS is within 0.5mph of the calculated reading. A spot reading on the GPS might be slightly out whilst vehicle speed is changing because of the refresh timing but at a steady speed the GPS shows a steady speed. Incidentally motorway markers are at 100 metre spacing.

 

[Deleted member 57092]

I built a rally car back in the day and we fitted a lower ratio back axle and didn’t bother changing the speedo gear in the gearbox.
Anyway at 6500 rpm in top this thing was doing 85mph which was sufficient in the Welsh lanes but the speedo said over 100.
A few years later it had been sold on a few times and had become someones daily driver with a legend built around it locally how it would pull 100 mph anywhere
Only it didn’t really
Nobody really wanted to know that though as it spoiled so many tall stories......

 

[tonyidle]

Good point
Forgot that
And tyre pressure
Atmospheric pressure
Ambient temperature
And wether the tyre has air in it
Or some boy racer has filled it with nitrogen
(Air being 80% nitrogen anyway)

Tyre pressure has no effect on speedo accuracy: the tread remains the same length even when it's flat at the bottom .

 

[Deleted member 57092]

Isn’t rubber stretchy then ?

 

[pappajohn]

You think? A true 65mph read 72mph on the speedo on my last (Hymer) van. The speedo was replaced just before I collected the van (either a recall or faulty) and I suspect that it, and probably the original, were intended for 16" wheels. For some unknown reason Hymer had specified 15" wheels on a Maxi chassis.

But that's a mechanical mismatch.
Had the correct sender been fitted it would have been much closer.

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[obanvanman]

Being an ex Traffic Police Officer may I offer some sound and sensible advice based on the not insignificant quantity of cheap gin consumed in Spain this evening.
When driving along and one of the natives flash their headlights to warn you of a forthcoming speed trap....put your foot down and then compare your Speedo reading with the nice officers reading....
Simples.

 

[pappajohn]

Incidentally motorway markers are at 100 metre spacing.

But odometer aren't

 

[Deleted member 57092]

I know a chap that claims the police came to see him in his mot garage after pacing his ferrari in their helicopter, he claims that they told him to slow down in future as the speedo ? In the helicopter was reading 160mph but wasn’t acceptable in court as it wasn’t calibrated.
I have no problem believing he was doing 160
He is pretty wild.

 

[pappajohn]

Tyre pressure has no effect on speedo accuracy: the tread remains the same length even when it's flat at the bottom .

Put enough air in and the tyres end profile will be more like a motorbike tyre.... Sidewalls pull in, tread pushes out until the two are balanced then, if it hasn't already, it goes bang. .
Think top fuel dragster rear tyres, they expand by a foot or more.

 

[Deleted member 57092]

Probably a radial tyre won’t increase in circumference if over inflated due to the steel belting holding it in shape.
But a crossply would
And maybe a bias belted tyre (tire!) too.
Dragster tyres will be something odd no doubt
They definately do that.

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[Lenny HB]

Is the total milage displayed on my mh based on what the Speedo is doing, in

No, use to on old vehicles with mechanical cable driven Speedos.
With the modern electronic ones the Speedo is set read high as a legal requirement as it is not allowed to read less than the speed of the vehicle for the life of the vehicle. The odometer will be calibrated to a higher a accuracy give or take variations in tyres etc.

 

[bigtwin]

Fantastic debate.

But since every other speedometer similarly indicates a higher speed than reality, you’re not accumulating any ‘extra’ mileage compared with anyone else.

Ian

 

[Deleted member 57092]

I did a bit of chauffeur work for a chap based in monaco who has an enormous amount of money, (and an even greater amount of free time)
When I casually remarked that since we had put radial tyres on the Rolls (1964 phantom) the speedo was reading faster than the sat nav he became fixated on the fact that it was depreciating its value by clocking up more miles than it was actually doing.
I put forward the view (after some thought) that the more successfully completed (recent) milage it had ,then the more likely he was to sell it as there was some proof that it actually worked.

 

[obanvanman]

Fantastic debate.

But since every other speedometer similarly indicates a higher speed than reality, you’re not accumulating any ‘extra’ mileage compared with anyone else.

Ian

You are absolutely correct about Speedos recording higher than reality.
When I was a surveillance motorcyclist chasing a 911 North up the M1 on a ZZR1100 my Speedo was reading 155 but apparently, according to Herts Police in their Cosworth, I was actually doing only 144.
(Allegedly)

 

[Jaws]

The only speedo I have ever known to be even close to accurate was in an ex plod Rover P6.... mounted in the middle of the dash, it was a Smiths unit about 8" round...

Had a few bikes that I have fitted speedo healers units on that got them close'ish

But yes, the mileage recorded is higher than actually done, but come on, why worry

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