Speedo setting

[PAJ]

I've set 3975 ppm on the dash, now it matches my other car, 50mph on the speedo is 47/48 on the gps, so the Atom now shows maybe 4mph more than it did previously, an 8% change (or so)

P.S pretty sure 16's, aren't the Dymags the same ?

I'm happy to have sorted it, I'll shut up now, Hedge will be happy about that

[John Scherrer]

Tyre width is good to know.

The tyre wall height of a 205/50 profile tyre is 50% of the width, so in this case 102.5 mm.

So .. the diameter of the wheel/tyre is (2 x tyre wall height in mm) + the diameter of the wheel in inches.

So much for metrication !

[PAJ]

Right, so there's s pick-up on one of the wheel/hubs that counts rotations ?

[John Scherrer]

Yes

There's a small round magnet glued to the nearside drive shaft that counts revolutions .. sensor is mounted above.

[PAJ]

That's awesome, today I learned something, good stuff, thanks John

[Monza]

@autobackup
if i have asked for the width it's because with Dymags wheel which are 16" rear from the factory, the factory has 2 mounts possible : the traditionnal 225 tire and the optionnal 245 and the circomference of them are not the same. And i know the dash setting for the 245. 

[autobackup]

Monza   

That directly depends on the manufacturer and model of the tyre you personally use on your Atom and is simply not true for all manufacturers an/or tyres!

A tyre is quoted for example as : 225/50ZR16 (92Y) - where 225 is the tread width 55 is the tyre aspect ratio (This is the ratio of the tyre’s cross-section to its width, expressed as a percentage) 92Y is the speed rating and R16 the wheel (unshod alloy) diameter.

Unfortunately for your premise both 225 & 245 tyres are available in different aspect ratios (height of the tyre sidewall) eg 225/50, 225/55, 245/50 and 245/55 (ie 50% or 65% of the tread width)

So for your premise/assertion to be accurate any other Atom would have to using exactly the same manufacturer and tyre aspect ratio that you use on your Atom - otherwise there inevitably would be (admittedly small) speed errors created!

So - given that the overall diameter of a tyre' aspect ratio is a percentage of it's width - effectively means that 225/50R16, 245/50R16, 225/55R16 and 245/55R16 ALL have slightly different overall diameters with tyres fitted

[Monza]

and you imagine i learn something from your answer that i don't already know ?? 
The factory fits only 2 rear sizes, perhaps you don't know them 

[autobackup]

Monza

Yes I would agree that what you say would be true but ONLY if the Ariel factory was the sole supplier of tyres for the Atom 

- however not every Atom owner buys their tyres from the factory as there are many other tyre manufacturers that make tyres eminently suitable (often cheaper) for use on an Atom on both road and track - e.g. Dunlop, Avon, Pirelli (Troffeo), Continental (road use only) just to name a few!