car mph versus tomtom mph, why different?

Car_54

George_Michael wrote: »

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



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

The implementing legislation in this case is the Road Vehicles (Construction and Use) Regulations 1986, as amended, as quoted in my post, and which are still in force.

There appears to be a conflict. However, the C & U Regs also require the speedo to be "maintained in good working order", and it is the C & U Regs, and not the type approval ones, which Mr Plod would apply at the roadside. An unlikely event, I suspect.

vaio

mcpitman wrote: »

Tyre wear will also mimimally affect speedo accuracy.

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

Etc etc.

my calculator says that with 185/50 15 tyres the engine revs needed for 60mph on new tyres only gives 58 mph on worn tyres

On the sat nav side, certainly sat nav can work out altitude and I'd be surprised if the mapping data wasn't in 3D too so I'd expect sat nav mph to be accurate even if you are going up (or down) hill

mcpitman

vaio wrote: »

my calculator says that with 185/50 15 tyres the engine revs needed for 60mph on new tyres only gives 58 mph on worn tyres

On the sat nav side, certainly sat nav can work out altitude and I'd be surprised if the mapping data wasn't in 3D too so I'd expect sat nav mph to be accurate even if you are going up (or down) hill

Yup and the variation in tyre pressures across wheels and variation caused by operating temperatures etc.

Strider590

If you look at:

http://www.willtheyfit.com/index.php?width=185&aspect=60&diameter=14&wheelwidth=6.5&offset=20&width2=195&aspect2=55&diameter2=14&wheelwidth2=6.5&offset2=20#content

You'll see that 7mm change in diameter equates to just 1.32% change in speed.

The average new tyre has 7mm of tread over the legal limit of 1.6mm.

This will of course increase with larger wheel sizes, but essentially the difference is very little.

IamNotAllowedToUseMyName

My observation is that over a distance the GPS is reliable as a general indication of speed but it is prone to glitches, so you cannot rely on it at any one point.

What I've noticed is that the GPS may drop out for a little bit, due to trees, bridges and so on, and then catch up. So on a clear straight road you can use GPS to safely calibrate your speedo, you'll get a good reading and maintaining an average speed, but if you watch it closely, you'll find that as your speed varies, the GPS does not always accurately track the speed you are doing. You can usually rely on it through a motorway average speed section, and using this and the knowledge that ACPO guidelines will enforce only at 10% + 2mph (i.e. you need to do a real 57mph to get a ticket), driving to an indicated 50mph on a GPS and say 53mph on the speedo means that you are on a fairly safe bet that you are doing 50, and you still have 6mph in hand.

The GPS can always find its current position after a glitch, but a speed reading is based on the distance between two points, and there can be minor variations from reading to reading.

The GPS is designed to be accurate enough for navigation, but the speed reading is a nice to have and you should not rely on it, say to check that you are doing 34mph through a speed camera.

My Vito van under-reads horribly, and I've calibrated it using several different GPS's and am confident that an indicated 34mph is actually a real 30mph - that's a big enough difference in speed to make me drive to the calibration rather than the indication to avoid annoying people. My other cars are far more accurate, my very old Merc was only out by about 2mph at 70.

shopaholicz

Thanks for all the replies. My car is only just 1 year old, so tyre wear I hope won't affect anything. So basically what everyone is saying is that the car mph is probably a little higher that I was actually doing and is meant to be that way, so tomtom probably most accurate of the 2. Interesting why the difference. Think I lost our debate as I said the car would be the truer reading. Haha.

I even got the bit about going up/down hill not making a difference to mph!! Thanks for the easy explanations.

JustinR1979

Yeah as said satnav is spot on - it always matching my tacho which has to be spot on.
I have found newer cars are about 3mph over at all speeds, and older cars 10mph over at all speeds - older cars have cable driven speedos.


Also explains why I have to overtake so many cars, probably sitting at 56 on their speedo as this figure is quoted as best mpg in handbooks, and they're truly doing 53mph.
If you get overtaken by a lorry, just hand your licence back, it's shameful.


I've noticed some of those signs that flash your speed account for this over read, as they flash up a higher speed than the tacho shows, telling everyone you're speeding when you're not.

bod1467

My speedo to satnav comparison table is pretty much as below:

32 .. 30
43 .. 40
54 .. 50
64 .. 60
75 .. 70

Paul_the_Painter

If we are assuming an allowable error of -0% to +10% it's worth bearing in mind that from an engineering standpoint that's difficult to manufacture. Most manufacturers would find it much easier to make a speedo that reads -5% to + 5%. So therefore they would instead make the speedo read if "perfect" at +5% , with their manufacturing allowable tolerances/errors of -5% to +5% that translates to -0% to +10%.

The result of which is that even if everything is lovely and new and working perfectly, speedos overread by about 5% - which they do (in general, don't come crying to me with a speeding ticket!)

JustinR1979

Maybe in the speedo driven cable days, just whack a few % on top.
With abs sensors used, can just add a couple mph at any given speed easily enough