Just got a GPS receiver, so I've been doing the obligatory comparisons between the speedo in my MY02 WRX wagon and the speed as indicated on the receiver.

Surprised to see that an indicated 100 mph (on that well known private track that we all use) is only a true 91 mph.... :(

Slightly less surprised to see that an indicated 30 mph on the speedo is a true 27 mph. 60 mph is a true 54 etc....

So, the speedo error seems to be reasonably linear - ie. the error is approximately the same in percentage terms, no matter what the speed.

Anyone else tried comparing GPS with speedo MPH - how accurate are you?

Road Angel, UK00 with 215/40x17 tyres which over-read by an extra 1.2%. 30 = 27, 70 = 66, 100 = 95, 130 = 122.

Hi Hoppy - seems more accurate than mine.... I'm starting to assume that my speedo is a blatant liar :)

Or, can these GPS units misread at all???

My Snooper 6 Neo gives similar results on my MY94 JDM car. I think the GPS speed should be pretty accurate as far as steady speeds are concerned. It just takes a little time to work nout the new speed if you change of course. At least the speedo is on the safe side anyway, as far as points is concerned.

Les :)

[Edited by Leslie - 05/06/2003 09:59:50]

Very true Les - but I was just 'upset' that my privately attained 100 mph was only 90-ish.... What does that make my privately achieved 140 worth the week before? ;)

Cheers, HB.

What wheel & tyres are you using ??
This could possibly be related to rolling circumference of wheels.

Hi Mate - just completely standard 17 inchers, with stock Potenza 215s. Typical MY02 boggo in fact.

my STiV's accuracy is really bad, according to the speedo im doing 100 mph on the M/Way in the morn & im not really passing anyone, either that or everybodys doing a ton, I'll have to change in to third gear me thinks :D

I used to get that with a modded Polo :) with 14 inch wheels and 45 series tyres. Even mountain bikes used to pass me at an indicated 60.....

i know speedos are not allowed to underread but overread by 10%. is this also linked to the odometer? i.e. does this mean a 100k mile car only has 91k miles :D?

Or that IM are overcharging us on servicing - ie a 7,500 mile service at only 6,750 :) :) :)

i just sva,d a scoob and the speedo was totally accurate at 70 it was doing 70 ...... lol...

oops

[Edited by wrrjones - 05/06/2003 12:16:03]

Harry,

I've just got a GPS that plugs into an Ipaq running TomTom. It too shows that the speedo is almost 10% over. I understand that it is common for most cars to be 10% over - this ensures that manaufacturers can never be accused of causing speeding offences or even accidents.

Hi Fazack - that's exactly the set up I've got. Have you visited the Tomtom site to get the latest software revisions?

Fazack - how many satellites are normally visible on yours? I usually get 6 or 7 - but have detected up to nine on the A1, M1 and M62.

Yes I know how you feel about what speed you really achieved HB, and I am wondering exactly the same about how fast I really got up to on an old runway. have to try it again with the GPS I suppose.

Les :)

I believe that it is more likely that the gps will be wrong, considering the small delay in sending and recieving signals from satelite (or however they do it). I would assume that if they can be a couple of meters out on positioning surely they could be out with calculating speed?

Could be Ian - that's what I'm trying to establish. Otherwise, what's the point of having a WRX that only does 60 in seven secs, or 100 in 20... ;)

Ian,
The time delay is pretty negligable(radio waves travel at the speed of light) and modern GPS units lock onto 4 or more satellites (3 for triangulation plus 1 or more for accuracy and backup). The one thing GPS is rubbish at is height because the time delay difference between 100ft and say 25ft is so small it's very hard to be accurate. Lat and long is easier because of triangulation.

At least that's how I understand it to work.

Sounds plausible Neil - so I HAVE got a lying speedo.... :(

The only reason I say that is because every car I have been in with GPS, freelander, beamer and renault. The speedo has always clocked the car at about 10% difference form the GPS.

It would be nice to get some confirmation on this, I am sure I've read that most cars are 10% over; imagine if a car speedo was under - the manaufacturers would have their a??e sued off them by everyone with a speeding ticket.

Ipaq5450 + Navman3000 + Tom Tom II .

MY01 WRX always 12% overreading ! .... Which is not too bad ... however now I know it over-reads I am tempted to go faster :D

Scott

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MY 01 WRX, 18" wheels,
79 mph on GPS = 73 on speedo.
30 mph on GPS = 27 on speedo.
Snooper S6R Neo.

I have the road angel and it is about 10% out:O)

I thought ALL cars were like this...it's the law?

Hi Scotty - mine's the Ipaq 5450, plus TomTom 2. About 10% overreading....

lets face it !!, the satalite has trouble keeping up with a scooby :P

That's what I thought Monstermunch. :) :) :)

Can't you check how accurate the speedo reading is by using the indicated engine revs and the gear ratios to calculate the speed ?

I think it's something like 3000 rpm in top gear is 76mph and 4000 rpm is 100mph (on a classic scoob).