Scirocco Central

Simple question really - what's more accurate? your speedo or Sat Nav?

My TomTom is always reporting my actual speed as about 3/4mph less than my speedo!

your speedo reads low for good reason..

The amended Road Vehicles (Construction and Use) Regulations 1986 permits the use of speedometers that meet either the requirements of EC Council Directive 75/443 (as amended by Directive 97/39) or UNECE Regulation 39.[11]

The Motor Vehicles (Approval) Regulations 2001[12] permits single vehicles to be approved. As with the UNECE regulation and the EC Directives, the speedometer must never show an indicated speed less than the actual speed. However it differs slightly from them in specifying that for all actual speeds between 25 mph and 70 mph (or the vehicles' maximum speed if it is lower than this), the indicated speed must not exceed 110% of the actual speed, plus 6.25 mph.

For example, if the vehicle is actually travelling at 50 mph, the speedometer must not show more than 61.25 mph or less than 50 mph.

http://en.wikipedia.org/wiki/Speedometer#United_Kingdom" onclick="window.open(this.href);return false;

hence the billy bs 0-60 speedo times we see, in reality 0-less than 60

I do think that a satellite that can position you to within 2 feet from an orbit around the earth is likely to be far more accurate than your speedo that is deliberately set about 8% optimistic. Ever driven past one of those "flash your speed up and shame you into slowing down" displays? Go past one of them when your speedo says 30, and you can guarantee your speed will be flashed up as 27-28mph.

Surely in 2011 a cars spedo is going to be far more accurate than a gps reading after it having to be translated through different receivers ??

kane1210 wrote:Surely in 2011 a cars spedo is going to be far more accurate than a gps reading after it having to be translated through different receivers ??

Not in my experience

I would of thought the GPS is reporting less than your speedo due to the delay in signals received from the satellite... But clearly not.

GPS is more accurate than a speedo but still not 100% accurate due to the fact it's calculating the speed on a moving target on a different axis.... a head on laser is the most accurate, irritatingly that being what them police folk have!

Although not really that relevent for driving with GPS, more mountain biking or hiking, but GPS speed and distance is basically done in 2D. I.e if you are driving up/down a big steep hill, the speed/distance will not be as it actually is, and would probably register faster that reality.

On hand held GPS, you would think that, as they calculate your altitude too, that they would also use this to calculate the actual spped too.

I went for speedo as like many who have been charged with speeding and say they use their sat nav device never gets away with it.
speedo aren`t 100% but its better looking at that than a number somewere else that you have to keep your eyes off the road

maisbitt wrote:..... Ever driven past one of those "flash your speed up and shame you into slowing down" displays? Go past one of them when your speedo says 30, and you can guarantee your speed will be flashed up as 27-28mph.

That's exactly what I have found, on several occasions.

How is a poll going to find the answer to the question? It's not a matter of opinion.

Let's be scientific about this and do some experiments with a passenger timing between 100m markers on a level stretch of motorway, over as long a distance as you can go at a steady speed. Cruise control may or may not be more accurate than your right foot; we'd have to do a different experiment to be sure. Multiple timings at different speeds would needed to check that both instruments have a linear response to changing speed.

My gut feeling - GPS FTW. A system that has been designed to take into account relativity effects is probably more accurate than a 5p pulse timer in an electrically noisy environment. Over a long enough distance I think the GPS will give the more correct average speed.

Note I'm not saying that relativity effects will kick in materially as you hurtle around the M25; rather it points to a well-engineered system where pinpoint knowledge of your location at a particular time was the primary goal, as opposed to a speedo which aims to give a reasonable picture of speed and never show a speed less than the vehicle is travelling.

My sister's fiancée is a traffic officer, and on the Volvo's, Focus ST and BMW X5 the tom tom they have says the same speed as the calibrated camera where as the speedo is always out! that's his opinion! He does say though, you can not justify using a sat nav as a speed indicator as they are not calibrated either.

My Old ST was set "up". When driving at say 50, it was really doing like 48. This is why with the 10% plus 2mph I happily drove up to 55mph in 50 zones and 80 on the motorway.

When I went into test mode on the motor, the digital read out confirmed this by saying 72 when I was doing 75

Of course if the speedo is reading high then that would explain how some peeps get rather optimistic 0-60 timings for their Rocs if they are relying on the speedo reading.
They might actually be timing only 0-55 mph. Apart from the fact that official figures are times for 0-62 mph as well.

GPS will be more accurate but other factors play a part like if you have changed the size of your alloys or tyres this will automatically alter your speedo accuracy, if you are not using a GPS device and have changed your stock alloy/tyre set up you need to be mindful of this.

If my facts are correct it is illegal for a speedo to underread, so manufacturs err on the side of caution and calibrate to slightly overread to avoid any problems.

ZephyR wrote:Of course if the speedo is reading high then that would explain how some peeps get rather optimistic 0-60 timings for their Rocs if they are relying on the speedo reading.
They might actually be timing only 0-55 mph. Apart from the fact that official figures are times for 0-62 mph as well.

totally agree, even better when they are from a rolling start, when we clocked 5.56s to 60 on my manual we used a racelogic box