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Speedometer Over-Reads
mark_n
 

Posts: 1,720
Joined: Jul 2011
Location: London
Post: #1
Speedometer Over-Reads

Nothing new there of course but the error is more than in my other cars.

I drove today with a GPS based speedometer in the car. With the speedo reading 83mph, the GPS device (which is much more accurate) read 78mph.

Mark

Evoque was great, now in an RRS SVR
24-05-2012 12:42pm
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speary
 

Posts: 778
Joined: Oct 2011
Location: Bolton
Post: #2
RE: Speedometer Over-Reads

Checked mine and its 1.5 to 2mph above actual speed
I am on 19" wheels withh 55 ratio tyres
Do 20" wheels with 45 ratio tyres have the same circumference ?
Same as my last Audi funnily enough

Pure Coupe SD4 Manual; Fuji White; 19" style 5 wheels; Addaptive Xenons; Auto Lights, Wipers and Rear Mirror; Folding mirrors with cool puddle lights. Magnoflow Exhaust finishers
24-05-2012 12:54pm
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mark_n
 

Posts: 1,720
Joined: Jul 2011
Location: London
Post: #3
RE: Speedometer Over-Reads

Yes, should have mentioned mine is on 20" 45 profile tyres.

Mark

Evoque was great, now in an RRS SVR
24-05-2012 01:00pm
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Straydox
 

Posts: 583
Joined: Nov 2011
Location: Warwick
Post: #4
RE: Speedometer Over-Reads

(24-05-2012 12:54pm)speary Wrote:  Checked mine and its 1.5 to 2mph above actual speed
I am on 19" wheels withh 55 ratio tyres
Do 20" wheels with 45 ratio tyres have the same circumference ?
Same as my last Audi funnily enough

There are on-line calculators for this (rolling diameter is the critical bit) - you feed in the parameters and they will indicate potential speedo deviation. There is one here Rolling diameter calculator link

Delivered 15th May 2012, rejected and returned 4th Feb 2013 - SD4 5 door Prestige manual with a bucket full of rattles Mauritius Blue. Crazy Audi MY2020 RSQ3
24-05-2012 01:35pm
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benzina
 

Posts: 873
Joined: Sep 2011
Location: Australia
Post: #5
RE: Speedometer Over-Reads

The 245x45 size chosen by LD for the 20'' rims are actually about 6-7mm lower in profile then should have been,in order to maintain the overall average circumference of all the other rim size/tyre combo's in the range. Thats about 14mm smaller overall circumference compared to all the other stock rim/tyre size combination range. This would account for some of that speedo error i would think.
255x45 or 265x45 would bring it closer to the average.
I payed extra to get them to fit 255x45's on my 20'' style 7's for this reason, and for that ever so slightly beefier look.

2012 Dynamic SD4 Auto 5-Door / Indus / Black Roof / Vortex / AD / Style 7 Alloys / Panoramic Roof / Rear Vents / BSM / TPMS / Sat Nav
(This post was last modified: 24-05-2012 02:12pm by benzina.)
24-05-2012 01:36pm
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ED209
 

Posts: 1,128
Joined: Aug 2011
Location: durham
Post: #6
RE: Speedometer Over-Reads

All cars speedo's over read,.

They only have to be accurate to +/- 10% therefore manufacturers make them overread slightly.

If they under read that people would complain that they were getting caught speeding when the speedo was telling them they weren't.

This is not a fault, its perfectly normal.

td4 Pure manual in orkney grey, auto wipers/lights, front fogs, spare wheel. carmats4u mats, travall dog guard. Collected 30/03/12.
24-05-2012 01:54pm
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Ezandania
 

Posts: 309
Joined: Aug 2011
Location: Brunei
Post: #7
RE: Speedometer Over-Reads

Same here Mark, I have the 20" wheels as well, and with a Garmin GPS, the speedo reads exactly 5km/h more than the GPS.

2012 Evoque Dynamic Si4 Auto, Fuji White/Agility Dynamic Plus, 20" Style 6's, Panoramic Roof, Keyless Entry, Power Tailgate, Surround Cameras, 825W Meridian Sound, Adaptive Dynamics, Adaptive Xenon HIDs
2003 Mini Cooper S
1999 Honda S2000
24-05-2012 02:23pm
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PhilSkill
 

Posts: 3,880
Joined: Jun 2011
Location: Warwickshire
Post: #8
RE: Speedometer Over-Reads

At 78mph the speedo must read a minimum of 81.2mph as I believe the requirement is a minimum of 4% overread. Plus your inflation level of the tyres will also affect the accuracy.

MY12 Fuji Dynamic SD4 Manual, Pano Roof, Electric Tailgate with own Close module
[Image: 124529.png][Image: 1004887.png]
(This post was last modified: 24-05-2012 04:28pm by PhilSkill.)
24-05-2012 02:45pm
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evQ
 

Posts: 36
Joined: Mar 2012
Location: USA
Post: #9
RE: Speedometer Over-Reads

(24-05-2012 02:23pm)Ezandania Wrote:  Same here Mark, I have the 20" wheels as well, and with a Garmin GPS, the speedo reads exactly 5km/h more than the GPS.

What model of garmin gps you are using ?
Not to hijack the thread as this is unrealted to speedometer, but I need to find a gps that works ( for navigation ) and would be nice to know what model you use.
24-05-2012 04:24pm
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evoquemayne
 

Posts: 86
Joined: Apr 2012
Location: London
Post: #10
RE: Speedometer Over-Reads

I don't know why, but there was always a part of me that never trusted my TomTom and therefore any GPS read out device simply because the map would always show me slightly behind where I actually was in real life. Therefore, I thought that my GPS was calculating my speed based on a (slightly) incorrect location and therefore a slightly slower read out would result. I guess the rate at which the location changes (speed) would still be the same though.
24-05-2012 04:36pm
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