Speedo accuracy.

4.0 V8 40v biturbo TFSI - 553 bhp
4.0 V8 40v biturbo TFSI - 597 bhp (Performance)
Post Reply
dannyuk
2nd Gear
Posts: 172
Joined: Sat Feb 16, 2019 1:56 pm

Speedo accuracy.

Post by dannyuk » Mon Mar 25, 2019 6:45 pm

Hi,

Every car I have owned before the speedo was always a little bit over the true speed (GPS) - seems on my RS6 its spot on. Is this standard on all RS6's or did previous owner code it?

Danny
2020 C8 RS6
2019 F90 M5
2017 C7 RS6
2016 F10 M5
2010 C6 RS6

User avatar
Daveperc
Cruising
Posts: 2582
Joined: Thu Dec 07, 2006 10:48 am
Location: Sandhurst, Berks

Re: Speedo accuracy.

Post by Daveperc » Mon Mar 25, 2019 7:19 pm

First question is what wheels and tyres you are running??

On OEM wheels and OEM standard Dunlop tyres mine read somewhere around 1.5-2% higher on the digital speedo than on my GPS - 1-2 MPH at 70.

On my (bigger) 295/30 tyres the difference has about halved so it's no more than 1 MPH at 70.

Dave

dannyuk
2nd Gear
Posts: 172
Joined: Sat Feb 16, 2019 1:56 pm

Re: Speedo accuracy.

Post by dannyuk » Mon Mar 25, 2019 8:29 pm

Hi Dave,

I am on 285/30 Continental. Interesting about the difference when going up a size.

Danny
2020 C8 RS6
2019 F90 M5
2017 C7 RS6
2016 F10 M5
2010 C6 RS6

User avatar
MikeFish
Cruising
Posts: 15588
Joined: Tue Aug 31, 2010 9:23 pm
Location: The Middle of Somewhere

Re: Speedo accuracy.

Post by MikeFish » Mon Mar 25, 2019 8:53 pm

I find the digital speedo is normally spot on or near as damn it. My thoughts; The normal speedo error is due to the analogue dials. The car knows exactly how fast it is going but has to over read on the dial slightly as it is illegal to show a slower speed than you are actually doing. Due to manufacturing tolerances between dials the easiest way to avoid this is to make sure it over reads slightly.

dannyuk
2nd Gear
Posts: 172
Joined: Sat Feb 16, 2019 1:56 pm

Re: Speedo accuracy.

Post by dannyuk » Mon Mar 25, 2019 8:55 pm

Hi Mike,

Definitely possible, I have an M5 and that is digital but shows 76 MPH when I am doing 70MPH on GPS!
2020 C8 RS6
2019 F90 M5
2017 C7 RS6
2016 F10 M5
2010 C6 RS6

User avatar
MikeFish
Cruising
Posts: 15588
Joined: Tue Aug 31, 2010 9:23 pm
Location: The Middle of Somewhere

Re: Speedo accuracy.

Post by MikeFish » Mon Mar 25, 2019 9:00 pm

dannya32 wrote:
Mon Mar 25, 2019 8:55 pm
Hi Mike,

Definitely possible, I have an M5 and that is digital but shows 76 MPH when I am doing 70MPH on GPS!
Wow, that is quite a way out. Even my B7 RS4 had a spot on digital speedo (only visible when using software with an OBD dongle). I had a tablet which displayed bit ECU speed and GPS speed and they were never more than 0.2 mph different (other than on fast changes of speed for the GPS to recognise). But the dial had the usual over reading errors.

dannyuk
2nd Gear
Posts: 172
Joined: Sat Feb 16, 2019 1:56 pm

Re: Speedo accuracy.

Post by dannyuk » Mon Mar 25, 2019 9:05 pm

Interesting, I prefer it being spot on anyway!
2020 C8 RS6
2019 F90 M5
2017 C7 RS6
2016 F10 M5
2010 C6 RS6

User avatar
Covkiller
Top Gear
Posts: 2349
Joined: Fri Mar 28, 2008 3:44 pm
Location: Coventry

Re: Speedo accuracy.

Post by Covkiller » Mon Mar 25, 2019 9:24 pm

The chap I bought my E36 M3 off owns a Vehicle Telematic company in Nottingham and sells to the big guns in the industry. He tells me that the GPS's in phones, computa's etc are a bag of <beep> when it comes to true GPS accuracy. Believe what you want but to me, if your RS4 B7 or RS6 C7 speedo tells you 194MPH, then that's what you're doing.
Last edited by Covkiller on Fri Jun 14, 2019 9:50 am, edited 1 time in total.
RS4 B7, Mugello Blue, K&W V3 & HR ARBs, set up by Centre Gravity, Milltek Valved Non Res & cat bypass pipes, BMC air filter, braided hoses, DOT 5.1, Terraclean, MRC Full Carbon Clean, MRC remap 448PS 477NM, 20" S7 alloys with 4mm spacers & Hawke spigots 66.6 > 57.1, AP Racing front discs. Sold Sept 2019. Now rocking an F80 M3

Permituk
Neutral
Posts: 3
Joined: Wed Feb 27, 2019 7:03 pm

Re: Speedo accuracy.

Post by Permituk » Thu Jun 13, 2019 10:25 pm

On my BMW there was a coding parameter to change this, default is 5% over but it was easy to code to be spot on. Haven't seen the same (also haven't looked :wink:) on my Audi but definitely is set to be accurate which nearly got me done when I was generous on setting my cruise control assuming a margin of error.

User avatar
GrahamS4
Top Gear
Posts: 2222
Joined: Mon May 08, 2006 7:41 pm
Location: Bucks
Contact:

Re: Speedo accuracy.

Post by GrahamS4 » Fri Jun 14, 2019 6:17 am

I think it's the refresh rate that generally differs in a typical consumer end products, mobile phone, portable SatNav etc. That is what is fundamentally different in something like a RaceLogic device. So at a consistent speed, there is less to tell them apart.

My B8 S4 on factory 19s has always been bang on what GPS states, sometimes will read 1mph under (probably only tenths in it).
Present: 2010 Ice silver B8 S4 s-tronic saloon (standard)
Past: 1998 Agate grey B5 S4 saloon (MRC tuned)
Hers: 2011 Volcano red 8P A3 Sportback 2.0 TDI 170 sport quattro

User avatar
Daveperc
Cruising
Posts: 2582
Joined: Thu Dec 07, 2006 10:48 am
Location: Sandhurst, Berks

Re: Speedo accuracy.

Post by Daveperc » Fri Jun 14, 2019 9:03 am

AFAIK car speedo's work by counting wheel revolutions and then doing some maths, either digitally today, or with a load of analogue gears in the old days. The actual display these days is a step motor.

If the input is wheel revs, then tyre wear will always be a factor. The rolling circumference varies by roughly 2% between brand new and fully worn.

blue95
2nd Gear
Posts: 112
Joined: Mon Jan 31, 2011 5:24 pm

Re: Speedo accuracy.

Post by blue95 » Sat Jun 15, 2019 7:20 am

Modern cars are deliberately under calibrated by up to5% by manufacturers these days, so if you do get caught speeding there’s not much defence...trust me I found out the hard way

ab54666
2nd Gear
Posts: 147
Joined: Sun Sep 30, 2018 4:44 pm

Re: Speedo accuracy.

Post by ab54666 » Sun Jun 16, 2019 7:34 pm

mines spot on also, running 21" wheels.

Post Reply

Return to “RS6 (C7 Typ 4G) 2013-2018”

Who is online

Users browsing this forum: No registered users and 126 guests