car mph versus tomtom mph, why different?

shopaholicz

Hi,

Yesterday I had to travel from Sheffield to Swansea, and back again.

Can someone please explain why the mph reading in my car always showed a higher rate than the tomtom did. Between 2-5mph difference all the time.

Also, we travelled through 6 x 50 mph zones on the way there and 5 on the way back. I never speed, after having a few speeding tickets a few years ago and begrudging every penny of the fines, but would like to know if the average speed cameras would agree with the car reading or the tomtom reading? Huge debate yesterday and want to settle our argument. Did give us something to talk about on the way down though. lol.

jc808

car speedos are not allowed to underread, but often do overread

also sat nav cant account that well for inclines

4743hudsonj

The GPS is likely the more accurate reading. @ 60mph id expect the speedo to read about 3 mph over (so im actually doing 57mph). Older speedometers can be as much as 10% out (past this they're technically illegal) .

Strider590

jc808 wrote: »

car speedos are not allowed to underread, but often do overread

also sat nav cant account that well for inclines

This ^^

My speedo reads 33 at an actual 30mph and 76 at an actual 70mph.

Also, interestingly, connecting to the cars ECU via the diagnostics port, the cars ECU measures the accurate speed, but it still lets the speedometer itself over read by about 10%.

It's both a bonus and a curse, because I can drive faster through speed cameras, BUT on the downside it can be infuriating when behind some idiot doing 40 in a 60, because I know they're actually doing about 35mph, nearly half the posted limit.

Strider590

4743hudsonj wrote: »

Older speedometers can be as much as 10% out (past this they're technically illegal) .

Actually the speedometer only has to work, it's accuracy is not testable.
The law regarding 10% is only for type approval, which the manufacturers have to conform to, much just like the emissions rubbish added to cars, which can be removed (like EGRs).

Gloomendoom

Strider590 wrote: »

It's both a bonus and a curse, because I can drive faster through speed cameras, BUT on the downside it can be infuriating when behind some idiot doing 40 in a 60, because I know they're actually doing about 35mph, nearly half the posted limit.

It's also probably why you get people hogging the outside lane of the motorway doing a steady 63mph.

The speedo on my car is dead accurate which, unfortunately, gives no margin for error with speed cameras.

tgon

Makes you wonder why car makers don't just use GPS to calculate speed by default. Would be all standard then

Iceweasel

Truck speedometers/tachographs are pretty accurate - especially the digital ones - they have to be re-calibrated every two years.

Couple that to a speed-limiter at 56mph and you see why so many car drivers are under the impression that trucks exceed the speed limits.

Most of the time (if not all of the time) it's the car which has a high reading speedo.

For them to pass a truck they need to have around 63 to 64 showing on their speedo.

I tend to go by the Sat-Nav and ignore the speedo.

Running at a steady 70mph I often get fingers wagged at me by folks who are convinced that they are doing 70mph when it's more like 64mph in reality and their speedo is fast.

Quiet_Spark

tgon wrote: »

Makes you wonder why car makers don't just use GPS to calculate speed by default. Would be all standard then

Because GPS needs "line of sight" to work, go inside a tunnel and the signal is gone.

laidbackgjr

I'm not convinced GPS is more accurate, if I set the cruise control on my car the speedo holds steady, but GPS can fluctuate by 5MPH dependent upon gradient & quality of signal (tunnels / thick trees) etc.

I read somewhere about the effect gradient can have, on a very steep hill it can result in GPS being 20+% inaccurate (and showing slower than you are really travelling).

phill99

Car speedos have an element based of mechanical function in the reading of speed. Mechanical set ups are prone to wear, misfunction etc. Sat nav is based on a simple time/distance equation.