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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

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

Hi Dave,

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

Danny

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.

Hi Mike,

Definitely possible, I have an M5 and that is digital but shows 76 MPH when I am doing 70MPH on GPS!

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.

Interesting, I prefer it being spot on anyway!

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.

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 ) 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.

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).

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.

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

mines spot on also, running 21" wheels.