I've just borrowed a friends Tom Tom for Le Mans and it's the first time I've had Sat Nav in the car and have an observation.

I know std speedo's are crap, but I expected the BMW's to be vaguely accurate. According to the Tom Tom the speedo is under reading by a good 7mph and this is at speeds around 60-80.

So how accurate are Sat Nav speed read outs and how do they compare to proper timing gear and speed camera's.

I understood that Datron etc also used GPS. Does the Datron stuff have quicker sampling rates for more accuracy or is the Tom Tom reading pretty close and the BMW's speedo is actually crap?

Whats your experiences? Going by the Tom Tom I could be driving faster and not worry about it

Ta

 

Speedo under-reading? Not likely.
Has it different wheels?
Sat navs are spot on 100%, checked mine against all our tacho'd lorries.
My bmw is 2/3 mph over at all speeds I've checked, and that's a 95 - run off diff so more accurate than your old cable driven that are typically 10% over at all speeds.

 

My Garmin sat nav reads about 10% slower than the speedo readings as well?

I thought it was highlighting the safety cushion that manufacturers build into the speedos?

 

tomtom is very accurate if you hold a steady speed for a while. i guess you meant over reading? i.e. that your speedo shows 68, but the satnav shows 60? if not, there is something wrong, perhaps different size wheels as suggested?

 

Sorry, yes meant over reading

 

They build in over-read, be taken to court if it under-read.
10% is typical on older cars, modern abs/diff read are pretty accurate, but always read over, unless wheels/rolling radius has changed.

 

Paul - you got stock wheels?
Mine with new tyres (old 95 M3) reads 80mph and 78 on sat nav, so pretty good.

 

Yes, Std CSL 19's

 

My dad used to have a 5 series back in 2002 ish, that over-read by 10% as well. Personally I think it is a scam to make services come round faster! If your speedo over reads by 10% then you are going to get your car serviced 10% more (even with the smart service intervals I think!)

 

im sure the sample rate of proper timing gear is higher than tom tom!
tom tom seems to sample once a second? datron etc is far more accurate than that.

but they must all use the same gps sattellites? so would be pretty good i'd say

 

Yes tomtom is 100% spot on as its done by gps. My tomtom reads 78mph when the vans speedo shows 80 so not far out at all. Whereas i know my mates Audi A3 used to read 70 on the tomtom with the speedo showing 79-80! same with his Passat come to think of it. Most other new-ish cars/vans i know are only out by about 2 or 3 at the most, so maybe its a german thing if your bmw is a fair bit out too.

 

Road Angel's were originally quoted as being within 0.7% accurate when they were first released. Therefore in theory ALL GPS based devices including Sat Navs are around this figure.

 

No problem if you're not on and off the throttle every second then, but cruising.
They know your location within a metre, so accurate.

 

you're right, the sample rate of proper timing gear is higher than satnav. i would guess that tomtom is 10Hz though, even if it only updates on the screen at 1Hz. but as i said, if you are holding the speed steady over a good distance, then it will be accurate to within +/- 1mph

and yes they all use the same satellites, but some systems are more sensitive than others and have more channels so can theoretically be more accurate

 

gps accuracy is about 15m, so roughly about 1%

gps can be even more accurate with the right codes, down to about 30cm's, but these codes are tightly guarded and licensed

 

exactly, so if you've travelled 10 km at the same speed, that 15m error becomes pretty small

 

very true mate,, i was trying to say that they are very accurate!

 

Are you sure it's 15M and not 1.5M?
Is spot on to a house number, not 50 foot out ever.

 

theres loads of things that contribute to the accuracy of the gps, electronic inaccuracy being just one

 

sorry, my last post didnt really help

just reading a little, the standard civil channels can be improved down to 30cm's and the military codes down to a few mm's

 

All cars are setup about 10% over reading. The comment about servicing your car more often as a result is not correct since the odometer is correctly setup. If you travel at 60mph on the speedo for 10 mins, you wont cover 10 miles.

All big manufacturer cars will do this no matter how new. My brothers 56 plate audi 4a underread by exactly 10%.

Satnavs speed calculations are accurite to 0.001% I beleive and use a doppler shift from multiple satalites to work our your exact velocity (velocity not being the same as speed).

JAmes.

 

Well mine isn't 10% out, only at 20mph

 

JAmes.

 

Oh actually, my mates 1989 Firebird is only 0.1% inaccurite....

JAmes.

 

my Audi A6 speedo is Always about 10% about at all speeds vs garmin satnav

however my TT is out by about 2-3mph from 30 till about 100 then its spot on at about 110 not looked long enough at sat nav at higher speeds

Nige

 

My S2 speedo is more accurate than my BM!

 

how bad are cossies? obv cant speak for every cossie but just in general

 

10% is typical of old cable driven speedos.

 

[QUOTE=cossiegreg1;3351213]how bad are cossies? obv cant speak for every cossie but just in general[/QUOTE


at 50 on my speedo on the cos its 51 on tom tom, 100 is 103 on tom tom.

 

Standard rolling radius by looks of that pic...

 

Well the speedo on my lorry is calibrated to 85knh max and my TomTom says 53/54mph which is about bang on.

 

My Corsa reads spot on at all speeds to my mates Tom Tom. Seems wierd being right????