Hi,

My Rio is on average 3mph slower than the speedo shows (measured via sat nav). The Jazz is bang on.

How does the Octy do for accuracy with 17" wheels? (Does wheel size matter?)

Cheers

Yes over all wheel circumference matters and average is about 3 mph difference.

Oh yeah for instance, the standard O3 wheel/tyre is 205/55/R16 @ 632mm diameter, change them for the 17" and you've got 225/45/R17 @ 634mm or a +0.32% difference so, the speedo will read a very small amount low unless its recalibrated, the 18" are even worst @ 225/40/R18 being 637mm or +0.79%.

Oh yeah for instance, the standard O3 wheel/tyre is 205/55/R16 @ 632mm diameter, change them for the 17" and you've got 225/45/R17 @ 634mm or a +0.32% difference so, the speedo will read a very small amount low unless its recalibrated, the 18" are even worst @ 225/40/R18 being 637mm or +0.79%.

Further more a new tyre with 8mm tread depth against a worn tyre down to 1.6mm.

 

Going by the middle 225 @ 634mm new goes down to 621.2mm worn, a decrease of exactly 2.00%. Another factor to take into account.

 

http://www.thecarexpert.co.uk/how-accurate-is-a-car-speedometer/

Edited 10 January, 201511 yr by cnc

Most cars over read on the speedo by a couple of mph. Don't see it as an issue myself.

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Andy, maybe thats why speedos read over so as, when the tyre are at their legal limit they read accurately !

 

I dunno, its just a thought.

Mine shows 151 km/h at 146 km/h on new 17's.

Measured with phone GPS and once by the police

There's a Construction and Use regulation or something that says a speedo can't under read but can over read by up to 10% + 4km/h within a specific speed range. My old octy 2 read about 4 to 5 mph high, so I knew I could do an indicated 75 on motorways and be fairly sure I was only doing a true 70. I've not been able to check my new O3 yet.

On my Superb when the cruise is set to 70mph on the digital readout it was recorded doing 68.4, so pretty good.

 

My car is fitted with a Quartix (http://www.quartix.net/) and the 68.4 came up on four consecutive days so it also shows how steady the set speed is maintained. I'll try to remember to check just before and just after having new tyres fitted to see what difference it makes.

Most cars over read on the speedo by a couple of mph. Don't see it as an issue myself.

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Depends on your speed. More than a couple of mph at 70

There's a Construction and Use regulation or something that says a speedo can't under read but can over read by up to 10% + 4km/h within a specific speed range. My old octy 2 read about 4 to 5 mph high, so I knew I could do an indicated 75 on motorways and be fairly sure I was only doing a true 70. I've not been able to check my new O3 yet.

My old Focus was always reading between 4 and 5mph high when at 70mph.

Andy, maybe thats why speedos read over so as, when the tyre are at their legal limit they read accurately !

 

I dunno, its just a thought.

 

Other way round, TMWNA - worn tyres rotate faster for a given speed so the speedo reads higher than when they are new.

 

According to my satnav (on new 225/40x18 Nokians) the speedo on mine is reading about 3% high (displays 70 when doing a satnav-indicated 68).

 

Also, tyre pressures can make a difference.  Over-inflate and the profile grows, let's say, a couple of mm.  Under-inflate the opposite happens.  You'd have to be well over or under to make a material difference though.

Based on my phone gps and radar detector, mine reads 4km fast at 100km/h. I'm on the 19s, but I assumed the wheel circumference on all the factory wheel/tyre combos would be the same, with the tyre height changing dependant on wheel size. Guess there are small differences though.

Thanks for the info!

It mostly matters in those average speed check roadwork zones. The lorrys always seem to be going faster than 50 though, and that's 50 on the Garmin!? (In a 50 zone before you ask!) Is there a 10% grace amount?

The old police mantra was 10% + 3.

Thanks for the info!

It mostly matters in those average speed check roadwork zones. The lorrys always seem to be going faster than 50 though, and that's 50 on the Garmin!? (In a 50 zone before you ask!) Is there a 10% grace amount?

At 50mph, I can set the cruise to 52 to maintain 50 on the GPS. To maintain 70 on the GPS, the cruise needs to be set to 72 and the GPS sometimes increases to 70 on slopes before correcting, if I set it to 73 on the cruise it seems to fluctuate 70-71 on the GPS. On the flat its OK set at 73.

Regarding the speed camera buffer zone, the attached is a guide, but not sure if the roadworks average speed cameras follow this which would suggest you get a ticket at 57 in a 50 zone...

http://www.which.co.uk/cars/driving/driving-advice/dealing-with-speeding-tickets/speeding-fines-your-rights/

Edited 11 January, 201511 yr by Matt Pez

Regularly travel 50 average speed check zones 56 on cruise. No ticket.

Regularly travel 50 average speed check zones 56 on cruise. No ticket.

I normally set mine to 54mph, no ticket for me either.

I went through a police speed check recently on a really long 30 limit stretch near my house, cruise was set to 30, digital speedo also read 30.....the sign they had set up said I was doing 31 and then had the cheek to flash a frowning face at me haha!

Im sure at 30 the cars doing a real 28ish mph so can only imagine their equipment is either dishing out a real approximation or was poorly calibrated....or my car under-reads by 1mph at that speed!

I went through a police speed check recently on a really long 30 limit stretch near my house, cruise was set to 30, digital speedo also read 30.....the sign they had set up said I was doing 31 and then had the cheek to flash a frowning face at me haha!

Im sure at 30 the cars doing a real 28ish mph so can only imagine their equipment is either dishing out a real approximation or was poorly calibrated....or my car under-reads by 1mph at that speed!

Sounds like a dodgy speed check.

I normally set mine to 54mph, no ticket for me either.

 

10% plus 2mph should mean you're okay up to 57mph

10% + 2 is a recommendation by (I think) The Association of Chief Police Officers. It is not mandatory - some police authorities operate a zero tolerance policy. So +1 mph over the limit = a ticket in some areas.

Just checked and my memory is correct its a non mandatory ACPO recommendation only. Funnily enough there is an article in the Sunday Times today about this. Average speed cameras are being introduced by stealth on some motorways. There is a new hidden Hadec3 average speed trap at Clackets on the M25. Kent police refuse to say what grace (if any) they will apply to this trap. Locals believe it might be zero. So watch out!

Interesting.  There was a lot of kerfuffle recently about raising the M Way limit to 80.  The presenter's guest (IIRC a chief constable from some force or other) stated this was actually very little change as the current limit was realistically 79 now (10% plus 2) and the new limit would be zero tolerance.

 

Not 100% sure of the facts though - someone else may know better.

I was under the expression now that's it doesn't matter what wheel size you run on a car. That the speed is picked up by abs sensor, which has a fixed rotation that is already set at the factory in the ECU.

I was under the expression now that's it doesn't matter what wheel size you run on a car. That the speed is picked up by abs sensor, which has a fixed rotation that is already set at the factory in the ECU.

 

Well, yes and no. The bigger the tyre the larger the rolling circumference so the less (or more if you go smaller) rotations per mile.  The speedo measure rotations of the wheels and that translates to a displayed speed.

 

A worn tyre will rotate more times per measured mile than a new one. Different sized tyres will produce the same effect in various degrees.  This is why satnavs don't quite measure the same as the speedo (most of the time) as they are measuring true traveled distance versus tyre revolutions.