Speedo V Satnav

Stuartleah

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I have noticed that when I drive at 50mph on my speedo my Satnav says I am only doing 46mph.

Which is correct??
 
I tend to believe the sat nav rather than the vehicle speedo, as the speedo is reliant on optimal tyre pressure and rolling diameter.

I drive a large variety of cars, vans and trucks, and have found great variation in indicated speeds, especially when accompanying another car etc, the sat navy's all tend to agree with each other, ( I have even tried two sat nav's side by side (garmin and tomtom), they both read the same speed.

I have noticed Japanese cars tend to have the most "optimistic" Speedos, European vehicles appear more accurate, though these seem to have an error still In the region of 5-7percent.

Tachograph wagons appear the best though
 
correct

Stuartleah said:
I have noticed that when I drive at 50mph on my speedo my Satnav says I am only doing 46mph.

Which is correct??
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only car in all my many cars that read correct was mini cooper s "bmw" swopped it for a cooper that over read as normal.:wave:
 
A speedo is allowed to over read but NOT showing less speed than you are doing. So the manufactureres err just in case
 
SatNav. GPS accuracy is significantly better than 1mph at normal speeds. The speedo isn't really deliberately inaccurate, but can't take account of changes in tyre diameter owing to wear and different manufacturers' tyre diameters etc. For example, my Alfa (car!) came originally with the option of 15", 16" or 17" wheels. I don't think the speedo is any different. The required accuracy of a speedometer is -0+10%, plus 4 km/h. That means at an real 60mph your speedo could be reading 60+6+(4*5/8)=68.5mph. That's a long way off.
 
I find the sat nav so much easier to read and see then the speedo that I now tend to use it even when I know where I am going just for the read out
 
My van based on a LHD 2002 Fiat Ducato 2.8JTD is way out, if I want to do 40MPH I have to have the speedo at 80KPH which as everyone knows should be 50MPH, even if I know where I am going I use the satnav for accurate speed readings
 
richardstubbs said:
SatNav. GPS accuracy is significantly better than 1mph at normal speeds. The speedo isn't really deliberately inaccurate, but can't take account of changes in tyre diameter owing to wear and different manufacturers' tyre diameters etc. For example, my Alfa (car!) came originally with the option of 15", 16" or 17" wheels. I don't think the speedo is any different. The required accuracy of a speedometer is -0+10%, plus 4 km/h. That means at an real 60mph your speedo could be reading 60+6+(4*5/8)=68.5mph. That's a long way off.
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Oh dear...

You overlook something. Your car may have 15, 16 and 17" rims fitted but your tyres have a DIFFERENT aspect ratio. Means your OUTSIDE diameter of your tyre is always the same, no matter how big your rim is.

It's the second number on your tyre which gets smaller when the rims get bigger.

like 205/55/15 becomes 225/45/16 or 215/40/17. All these tyres have the same outside diameter and the speedo (and much more important your gearing) stays the same
 
I understood satnav measures your horizontal speed? If you were going down a hill your horizontal speed could be less than your actual surface speed, or speedometer reading, so an easy target for the speed cameras????
 
shortcircuit said:
I understood satnav measures your horizontal speed? If you were going down a hill your horizontal speed could be less than your actual surface speed, or speedometer reading, so an easy target for the speed cameras????
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Correct! satnav speed is more accurate than speedo, unless you are on an incline when there can be quite a variation, my truck tachograph (regularly calibrated) used to be nearly as accurate. nowadays i've got a digital tacho but a speedo that seems to be as inaccurate as the average car!
 
Teutone said:
Oh dear...

You overlook something. Your car may have 15, 16 and 17" rims fitted but your tyres have a DIFFERENT aspect ratio. Means your OUTSIDE diameter of your tyre is always the same, no matter how big your rim is.

It's the second number on your tyre which gets smaller when the rims get bigger.

like 205/55/15 becomes 225/45/16 or 215/40/17. All these tyres have the same outside diameter and the speedo (and much more important your gearing) stays the same
Click to expand...

I do understand tyres. Believe me, different aspect ratios notwithstanding, the wheels on this one (17" rims) fill the arches much more than on the last one (15" rims). Because, although the aspect ratio is indeed lower, they're also much wider, and the aspect ratio is a percentage. Sort of. And they still wear out, and the diameter can't help but reduce then, can it?

Maybe that was a bad example. Point is, the speedo can't take account of the many factors that can affect the diameter of the wheel and tyre combo. Wear, inflation pressure, load, temperature, tyre type (M&S or summer), all these things change the speedo reading.
 
shortcircuit said:
I understood satnav measures your horizontal speed? If you were going down a hill your horizontal speed could be less than your actual surface speed, or speedometer reading, so an easy target for the speed cameras????
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Yes and no. If the receiver has acquired sufficient satellites (at least 4, but preferably 6 depending on their relative position) then it can determine position and therefore speed in three dimensions. But the altitude calculation isn't quite as accurate (I think) so an incline will affect it. But it's still loads better than a normal speedo. Tachos are indeed much better then a normal speedo though, and the police calibrate the speedos in the traffic cars so they're good too.
 
Satnav is far more accurate than your car's speedo.

Satnav's are portraying the speed over land (which may be going up or down) over the Earth's surface and this is measured/calculated by the time differences between the receiver (in your vehicle) and the respective satellites positioned around the globe in geo-stationary orbits sending those signals. Vehicle speedo's (AFAIU) are actually permitted to be up to 10% in error (on the +ve side, not less than), this can takes care of the discrepancies between new and old (worn ) tyres and variant tyres fitted to one's vehicles.

DT
 
all speedo's are deliberately manufactured to over read the speed at a maximum of 7% [ used to be called grade B ] ... for an accurate test try having an observer of the mileage posts on a motorway comparing speed /time/distance ]
normal error is about 5% over which ensures that you never get an under read

as already stated sat nav's are more accurate but not perfect , they have no legal standing , but if you compare on a straight flat road you will get an almost perfect comparison of your speedo to reality

amazing what you learn if you spend your career with a tyre manufacturer !
 
GPS isn't very accurate for vertical distance because the satellites are so far away -- the top of Everest is about 0.04% of the way to the orbit of a GPS satellite. That also means that inclines have negligible affect on the reported ground speed.
 
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