I've got a new (used) car with aftermarket wheels on. They are the same size as the originals but would they have an effect on the speedo giving incorrect readings? For example at 30mph the GPS says I'm going about 27mph which is fine...but at say 90 I'm in fact only going 80...on a private road this was tested. Wrong tyre combo?

Any help much appreciated.

What size tyres do you have?

Not the wheels, but the tyres on the wheels make the circumference, total diameter.

http://kouki.co.uk/utilities/visual-tyre-size-calculator

 

What is the standard size fitment ot wheels / tyres and what options of these,   & what is the size fitted.

Is this on the Smart.

Does it have different size tyres front and back as standard from the factory?

Edited 7 August, 20169 yr by GoneOffSKi

I'll have to check. The aftermarket's have got wider on the rears. I'll check what the standard ones should be. So in theory if I out the right tyres on (even with a wider track) that should cure the problem

It is not a problem is it, the speedo over reads and does on many cars.

 

Now you know what the difference is you can go a little bit faster than the speedo shows.

 

Just check what the GPS is at 50, 60 & 70, and if you are doing 90 indicated on the speedo you are already breaking the UK NSL.

I've got a new (used) car with aftermarket wheels on. They are the same size as the originals but would they have an effect on the speedo giving incorrect readings? For example at 30mph the GPS says I'm going about 27mph which is fine...but at say 90 I'm in fact only going 80...on a private road this was tested. Wrong tyre combo?

Any help much appreciated.

If this is correct, then your speedo is over-reading illegally. It must not show a speed more than 10% higher than your actual road speed at speeds between 20 and 80mph.

 

Assuming your speedo already over-read "a bit", then even with same size wheels, if you've got narrower and/or lower profile tyres fitted, then you may have created this situation.

Actually is it not 'Must not show more than 110% of your actual speed'.

which is different from must not be 10% over.

Actually is it not 'Must not show more than 110% of your actual speed'.

which is different from must not be 10% over.

I'd read them both as "indicated must not be greater than 1.1 * actual".

Well i must be reading something different that was taken by the UK from the EU Legislation which is not the same as the Law.

So must not be 110% more than the actual speed.

Well i must be reading something different that was taken by the UK from the EU Legislation which is not the same as the Law.

So must not be 110% more than the actual speed.

Maybe I'm being thick here, but how is 100% + 10% of a quantity different from 110% of it?

It is not different in the answer you get, simple maths.

It is different in how you quote what 'they' say.

 

As to a Speed of 27mph and the speedo showing 29.7 or 30 mph is there any legal actions being taken by any Police in the UK, 

or the DVSA. 

Or if you are doing 67mph and have an indicated speed higher than 73.7.

Is the difference even picked up on a vehicle going in to a VOSA (DVSA) Testing Centre for a IVA?

They never have been on my vehicles.

 

At least those aware of there speedo being different from actual speeds might be able to adhere to speed limits and also proceed in a safe manner which should keep those officers of the law happy.

I think that you realise that you can't actually read a speedometer to decimal fractions of a mile per hour!

 

My original point was that, if your speedo is calibrated such that worn standard tyres give V_indicated as 109% of V_actual , and you then fit a worn lower profile your V_indicated could be something like, say, 112% of V_actual , which is then illegal even if the law is not enforced rigorously.

15 years now i have been running 29" and 31" tyres on JImny's that come as standard on 27" tall tyres.

Never changing the speedo drives.

But then i am just a rebel without a cause.

 

While you can not read a speedo to decimal fractions it is good on Skodas where when fitting bigger tyres you can compare the OEM size and bigger fitment with the Mileometer, and setting for distance covered and see the decimal fraction differences you can get over 100 miles or so.

Then easily compare when doing regular runs and checking the GPS for Speed Indicated, actual and distances.

So the car should have

205/40R17 on the front and

225/35R17 on the rear.

It's got

215/40R17 on the front and

245/35R17 on the rear.

Anyone got any idea what I need to do to get it right? I'm so confused..I like the width on the rear. I could change the wheels but as the arches have been widened at the rear to accommodate the wider wheels I don't want it to look odd.

How about leaving it as it is and remember to make the mental adjustment to allow for the speedo misreading?

Or is that not an option.

I could but the gap gets bigger the faster you go and it's hard to keep track. I currently have the trip computer on in the inbuilt satnav to help but then I can't use the rest of the stereo screen haha.

The roads that have speed cameras are mostly NSL which is 60 mph. and the Motorways are 70 mph.

Mark the Speedo with a bit of tipex.... Then you know your speed.

 

Seriously.

15 years now i have been running 29" and 31" tyres on JImny's that come as standard on 27" tall tyres.

Never changing the speedo drives.

As previously posted it's not about only wheel diameters, it's about rolling radius of wheel plus tyre.

 

On my other, non-Skoda, car I run OE 14" wheels in the Winter (fitted with Nokia Haakepelitta tyres) but 16" wheels in the Summer - but the rolling radius of the 16" wheel/tyre combo is LESS than that of the 14" wheel/combo (by less than 1% so the effect on the speedo accuracy is negligible).

 

So larger wheels doesn't imply larger rolling radius...

We kind of got that.

 

But the OP does have a Total Diameter/ Rolling Radius with bigger tyres, as do i in the example, 

and standard wheel diameters.

So for us it requires changing Speedo drive or other mechanical measures to get correct speedo readings. (some can be done with a computer.)

On some vehicles easy, on others more involved.

 

This is in post #3,

but i will post the link again.

http://kouki.co.uk/utilities/visual-tyre-size-calculator

Edited 14 August, 20169 yr by GoneOffSKi

So the car should have

205/40R17 on the front and

225/35R17 on the rear.

It's got

215/40R17 on the front and

245/35R17 on the rear.

Anyone got any idea what I need to do to get it right? I'm so confused..I like the width on the rear. I could change the wheels but as the arches have been widened at the rear to accommodate the wider wheels I don't want it to look odd.

Which is 40% of 10mm higher sidewalls in front (4mm) and 35% of 20mm higher (7mm) in back. That's about usable tread depth (based on UK law, not a recommendation to go down to the wear bars), so it shouldn't really be an issue, but we needed the standard and actual sizes to work that out!

try this website http://www.tyresave.co.uk/tyresize.html will tell you the % difference for the different tyres. Over reading shouldn't be taken as a problem, under reading is more serious. In my Mk1 the speedo at indicated 60mph is actually 56mph on the gps

The other issue here is that we're all assuming that GPS indicated speeds are somehow perfectly accurate, which is unlikely to be the case.

All the ones i have used are accurate enough to use through average speed cameras. 

They are accurate when checked against more expensive timing boxes on cars.

They tally with the Police that check Taxi meters.

A quick check is to drive towards a village road side speed sign. the Sat Navs i use are all dead on.