Hi all, just a quickie, what does your satnav read when speedo shows 80mph?

Mine shows 73 mph..................about right?

Si.

 

75 on SatNat = 80 on speedo for me.

 

It depends on the make of car and the level of over-read in the speedo. It also depends on what speed you are doing and whether the satnav updates it speed reading in real time or, say, every 2 seconds.

For me my GPS updates every 2 seconds. If I hold the car at 30mph steady for longer than that (not easy to do) to validate against the GPS unit my speedo over-reads by 2mph, ie GPS unit says 28mph. The assumption made here is that the GPS readings are accurate.

If I hold a steady 70mph then my speedo over-reads by 3mph, ie GPS says 67mph.

Hope this helps.

 

There's all sorts of tolerance stack-ups in the drivetrain system that can make the speedo "inaccurate" so that's why speedos are allowed to read up to 10% over but not under-read.

For example, as your tyres wear down, the speedo reads faster as you're effectively altering the gearing ratio...

gps is the most accurate signa you'r going to get in normal money terms as it's just calculating time and distance from the satellite to work out speed - no car inacuracies involved

As WB mentioned, gps only updates every second or so which means you need to hold a constant speed to compare the car and gps - there's no point trying to do 0-60 tests with sat-nav

My bugeye speedo was 10% out at all speeds but my STi is more accurate but doesn't seem to be as linear so I'll have to have an experimenting session

 

Most handheld, car and domestic systems use doppler shift rather than distance/time calculations. I am not aware of any consumer units that use the distance/time calculation though I am sure there may be some out there and maybe you do have one.

We had this discussion here some time back and there was one person who posted convinced that his GPS was doing a distance/time calculation. After some discussion, and to considerable amusement, it turned out that he was flying Hurcules transport aircraft and their GPS does indeed work out velocity in this manner. However, chances are any unit you will have access to will use doppler.

In the past, when SA was turned on, there was an easy way to prove this as domestic handheld units could give you a good speed reading before you had walked a distance equal to the average error introduced by SA. It is amazing the workings of a wheelbarrow, as they say.

 

At the risk of re-covering old ground; it'd be a mistake to presume that the GPS kit to which we commonly have access is as completely accurate.

Of course, as this is ScoobyNet there'll be someone along in a minute who will argue with me, whilst plainly knowing f*ck-all.....

 

Speedo 80 Satnav 73

Checked on three different satnav systems

MY02 WRX

Kiwi

 

Thanx to all who replied, with this in mind, it brings me to my second question which is.........................how fast is my scoob REALLY................??

Si.

 

Its not

 

In my Vectra it's only 2 mph out at that speed but my scoob is around 8 mph out

 

80 = 73 with me on both my TomTom mobile and my Road Angel.

 

At least it's not just my car

 

Same here using both on a MY00.

 

Hedgehog, I don't suppose you have a link to that old discussion post do you?

 

Afraid I can't find it, did try a search etc. but no joy. I'm pretty sure it was on here but I suppose there is a chance that it was somewhere else or even an email exchange. I have some vague recollection of us resorting to calculus at some point in the discussion, but maybe that was another discussion.

Do you think using GPS, or discussing it, rots your brain and causes you to lose your memory?

 

My Bugeye speedo read exactly 10% more than my road angel until I changed wheels/tyres. No I reckon it's a bit better, maybe 7% over.