car mph versus tomtom mph, why different?

RMS2

Curvature of the Earth (i.e. inclines and declines). Speedo records it, GPS doesn't.

Richard53

laidbackgjr wrote: »

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).

Gradients affect the GPS reading, but not as much as that.


Using Pythagoras, a journey of 100 miles up a 1:4 slope (about the most extreme example I can think of) means the car travels 103 miles up the hill for 100 miles horizontal travel. So the GPS will under-read by about 3%.


Taking a less extreme example, a 1:10 hill would make it under-read by under half a percent.

George_Michael

Strider590 wrote: »

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).

This is almost correct.
The legislation regarding type approval (The Motor Vehicles (Approval) Regulations 2001), actually state that the speedo can over read by as much as 10% + 6.25mph so if you are doing an indicated 70mph, you could in reality be going only 58mph.

Iceweasel

I agree that a sat-nav won't be accurate on a hill - but how inaccurate?

Even on a road that climbed 3000ft in half a mile the error would be about 1%

I'm no maths professor so feel free to correct me.

I reckon that is still more accurate than any other instrument I could purchase.

I have never seen my sat-nav vary compared to my speedo - it's a constant error.

Sat-Nav says 70mph - speedo says 73mph - uphill and down.

bod1467

With my cruise control set to 75 my satnav (whether Tomtom or phone app) will read 70, up hill and down dale.

Car_54

George_Michael wrote: »

This is almost correct.
The legislation regarding type approval (The Motor Vehicles (Approval) Regulations 2001), actually state that the speedo can over read by as much as 10% + 6.25mph so if you are doing an indicated 70mph, you could in reality be going only 58mph.

Not quite.

"The Road Vehicles (Construction and Use) Regulations 1986, as amended, allows the use of speedometers that meet the requirements of EC Community Directive 75/443(97/39) or ECE Regulation 39. Both the EC Directive and the ECE Regulation lay down accuracy requirements to be applied at the time of vehicle approval for speedometers. These requirements are that the indicated speed must not be more than 10 per cent of the true speed plus 4 km/h. In production, however, a slightly different tolerance of 5 per cent plus 10 km/h is applied. The requirements are also that the indicated speed must never be less than the true speed."

So, 10% plus 2.5 mph. The lowest possible real speed with an indicated 70 would therefore be 61.4

George_Michael

Basically, the info that you gave states that the EC directives can apply, not that they must apply. (it actually states that the Road vehicles regulations allow the use of a speedo that meets the requirements of the EC directive. It doesn't state or imply that all vehicles must meet those requirements).
There are many vehicles being driven in the UK that do not comply with the motor vehicle approval reg's due to them being designed, built and approved in other EU member states, hence the reason for their speedo not having to be approved directly to non UK legislation.

If a vehicle has been given approval in the UK under The Motor Vehicles (Approval) Regulations 2001 then the speedo accuracy can be as I stated (up to 10% + 6.25mph over the true speed.)

http://www.legislation.gov.uk/uksi/2001/25/schedule/3/made

3. For all true speeds of between 25 mph and 70 mph (or the maximum speed if lower), the difference between the indicated speed and the true speed shall not exceed— V/10 + 6.25 mph where V = the true speed of the vehicle in mph.

Don't forget that EC directives are not law as such and require implementing legislation

mcpitman

Tyre wear will also mimimally affect speedo accuracy.

New tryes @ 8mm tread, worn at 2mm tread, 6mm variance, 12mm overall (2x6mm).

Etc etc.

k3lvc

shopaholicz wrote: »

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.

I was slighty confused by my new TomTom this week when I drove through the 50mph limits on M1 - when I looked more closely it had picked up that I was in an average speed limit and was actually showing the average speed from the start of the roadworks. Impressive but slightly flawed as I could have increased significantly between a set of cameras and picked up a ticket for that stretch only whilst being within the limits for the whole restriction

reeac

laidbackgjr wrote: »

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).

To get a 20% error, assuming that it's due to a "cosine effect" [i.e. the road looks shorter to the satellite from above as it's sloping] it would have to be a VERY steep hill ...steeper than 1 in 2.