SatNav Speed?

[johnny]

As far as I'm aware nearly all speedos over-read....as in give you a higher reading. I could be wrong, but I think in Germany the car manufacturers are obliged to have no more than a 10% error...and I think thats more or less adopted by all manufacturers.

Anybody from an engineering background (me) will tell you that any type of measurement device will have an error. The less you want the error (ie more accurate reading) the more the sensor will cost. Ergo, car manufacturers try to use as cheap and as basic measurement device as possible.

Im not sure how accurate the GPS systems are mind you....

[colm_mcm]

car manufacturers overstate the speed because it eliminates risk of the speedo understating speed and creating all sorts of problems for themselves.

[kevinod]

Is there much in the difference between the 2?

[mcgon1979]

couple of those big signs on the M50 that say "YOUR SPEED IS ?? KPH" as you approach the 60KPH zone near the N3 exit. I have approached them several times at a steady constant speed and the speed they give (from radar) is always about 10% slower than my speedo reads.. I am on standard tyres and rims.

[ikeane]

This is all very interesting

I figure I already keep it barely legal, no point in pushing it...yet

[colm_mcm]

the faster you go, the more out the clocks go. so that top speed you've got on your car is actually a good bit lower than you thought!

[Sebastian]

100 on the clocks shows 97-98 on my satnav.

i'm not bothered .




on the |YOUR SPEED| signs shows lower than the dials as well 1-2-3 kms

i'm not bothered ...
Yet ...Is this a good way to trick the speed cameras or your mind ?

[colm_mcm]

your wheels are larger rolling radius than standard though Seb.

[Daz]

I have satnav in the car. When I'm driving along I can set it to tell me what speed I'm going. Thing is, it always reads slower than the speedo dial in the car. Question being. Which one is more accurate? I'm driving with standard wheels so that's not interfering with the speedo. It could be the difference of getting a speeding ticket or not. Anyone got any thoughts??

Sat nav is more accurate and don't forget to factor in wear and tear on the speedo linkages cogs ect.

Daz...

[colm_mcm]

The speedo on the FTO isn't mechanical, so no linkages.

Re: Sebs car, if it was on standard wheels with a smaller outside radius, the speedo would be reading 100kmph but the satnav instead of reading 97-98 , it'd be reading 94-95

[mcgon1979]

Does sat nav only measure lateral movement? If your driving up a 30 degree incline at 100kph the sat nav would only detect 80kph movement if it thought you were driving on level ground? Does it take that into affect too? Where as a read from the wheels directly is accurate regardless of hills.

[d_andrews78]

Does sat nav only measure lateral movement? If your driving up a 30 degree incline at 100kph the sat nav would only detect 80kph movement if it thought you were driving on level ground? Does it take that into affect too? Where as a read from the wheels directly is accurate regardless of hills.

Good point. Over to the physics boffins for an answer on that one. Any physics boffins on the forum??

But most Nat Nav or All GPS systems will also tell you the level as well.
some hand held gps locators claim that they can be to within +/-10mm in all directions.

[mcgon1979]

thats very good if thats the case. I didn't think they were altimeters as well as 2d lat/long locator's. If a car sat nav took all those points and calculated your movement based on the 3 dimensions it would be highly accurately and could even give you a real-time climb rate as you drove up a hill.

[colm_mcm]

Does sat nav only measure lateral movement? If your driving up a 30 degree incline at 100kph the sat nav would only detect 80kph movement if it thought you were driving on level ground? Does it take that into affect too? Where as a read from the wheels directly is accurate regardless of hills.

The new Pioneer head-unit satnavs have a tilt sensor so they can calculate distance travelled and speed better.

[d_andrews78]

I got my Dad a Garmin Sat nav last xmas out of halfords and it has that feature, think he's even tested it in a helecopter, he works on them (we don't own one). Think he said it wasn't far out.

[Daz]

[quote="colm_mcm"]The speedo on the FTO isn't mechanical, so no linkages.

/quote]

Ah well thats good to know, i haven't had to do anything with my speedo but have had to take some out of other cars in my youth so i was going by that, (on a side note i'm glad the old boys in blue don't realise that, i've used wear and tear to get off after being stopped doing 68 in a 60.)

Daz...