Speedo setting

[PAJ]

When I got my Atom I noticed that the speedo was miles out. Displaying 50 on the speedo, my tomtom (tried and tested on many cars and bikes) is showing around 52mph. Usually a typical car would be showing around 54 on the speedo when the sat-nag says 52. This is all approximate of course. Ariel showed me how to change the 'pulses per mile' as they thought mine had been set for the wrong tyre size. I still find the speedo to be reading 'under'..... but my small brain can't work out which way to change these 'pulses per mile' to make a given speed show a higher reading on the dash

Can anyone help ?

Many thanks

[Bruce Fielding]

In the UK, mass produced cars have to overread by 10% by law.

My Dash is 100% accurate according to GPS so I can't take liberties like I could with a standard car.

Logic would suggest that the more pulses per mile, the slower the speedo reading.

[Monza]

PAJ,
can update your profile and add the type of Atom you owned ? so that i can know the type of dash and help you for this setting 

[PAJ]

Thank you, I added some details yesterday. It's a 2016 car, so the latest dash

[Monza]

put the contact on
When Dash on, press the M button to enter the Menu
With the Up and Down Button, go to "Speedo and Rpm" and Press "S" button (Select)
Now go up/Down to "Pulses per miles" and press S button
now you can modify the value of some % depending of how much in % you need to adujst your speed or ask for the good value to Ariel with the tyre sizes you have.
Job done.
I'll send you a mail to.

[PAJ]

Thanks very much for the info and instructions. I already have Ariels correct setting, it's just not 'correct enough' and could easily get someone booked for speeding when they think they're just on the speed limit. I will experiment, and will post the figure that I think corresponds to a 'typical car' as well as a spot-on 'speed gun' or 'GPS correct' speed

[autobackup]

PAJ

Take care when calibrating your speedometer using GPS as GPS can suffer from local effects which can combine to produce significant inaccuracies as GPS speed is 'normalised' (i.e. averaged) over a period to smooth fluctuations (reflections) in the received GPS signal path.

Some stand alone car GPS's can only acquire data from 6 or 8 satellites - the best satnavs use data from 12 or more satellites.

To calibrate your speedo using GPS speed I would suggest using at least 2 separate GPS satnav units for comparison if you can

- if not you might consider using another car asking the driver to maintain a sequence of set speeds as you either lead or follow (if the car also has GPS even better!).

Currently you don't actually know which of your speedo or satnav is actually accurate as you have no means of checking either of them reliably with only one unit of each.

http://gpssystems.net/accurate-gpsbased-speedometer/

[Hedge]

Is 4% "miles out"? I don't really get it. If you go through a 50mph camera - or even an average speed section - at 52, you're not gonna get busted.

Just my 2c.

Cheers,
Hedge

[autobackup]

Bruce

Your post that 'In the UK, mass produced cars have to overread by 10% by law' is not actually correct!

This is the current law :

The law for car speedometers in the UK

The UK law is based on the EU standard, with some minor changes.  A speedo must never show less than the actual speed, and must never show more than 110% of actual speed + 6.25mph.  So if your true speed is 40mph, your speedo could legally be reading up to 50.25mph but never less than 40mph.  Or to put it another way, if your speedo is reading 50mph, you won’t be doing more than 50mph but it’s possible you might actually only be travelling at 40mph.

To ensure that they comply with the law and make sure that their speedometers are never showing less than true speed under any foreseeable circumstances, car manufacturers will normally deliberately calibrate their speedos to read ‘high’ by a certain amount.  As your satnav is not the designated device by which a car’s speed is measured, it does not need to incorporate any fudge factoring.

[Bruce Fielding]

[quote="autobackup"]
Bruce
Your post that 'In the UK, mass produced cars have to overread by 10% by law' is not actually correct!
[/quote]

Sorry, of course I should have said '...because of the law...'

Pedant? Moi?

[autobackup]

I took issue with your use of 'have to' read  -  not the bit about the law!! 

The actual requirement for car speedos in the UK is that the primary speed measurement device :

1. Must not underread by anything
2. May range from exactly accurate to an allowable variation of PLUS 10% (plus a further 6.25mph - which is a bit bizarre!!)

[PAJ]

I didn't get round to making any adjustment yet, but can anyone else either confirm the setting they have for the Dymag wheels option ? Or do they know how their speedo reads compared to a typical car (or GPS) ? And does it not worry Ariel they've got people travelling at a speed that's greater that the speedo shows ?

[Hedge]

Again, by 2 mph? I'd say probably not. But, hey, what do I know 

Cheers,
hedge

[Monza]

Which tire size ? 225 or 245 ?

[autobackup]

I think that you need to know the alloy and tyre diameter i.e. 15" or 16" alloys plus tyre depth (55?) rather than the width!!