Used RNS-E sat nav to verify 8P speedo

On the RNS-E sat nav system, there's a feature called Engineering mode (press and release the NAV button, hold down the upper left button from the main knob for 5 seconds, release, then hold the upper right button for another 5 seconds). Amongst all the visible satellite info and other measurements, I noticed one for speed, shown in both mph and km/h (I think it's under the General section). Obviously the sat nav can work this out for itself, based on your position and how quickly you move between satellites.

Anyway, I thought it would be neat to find out what differences there would be between this readout and the speedometer. Speedos are renowned for over-reading - I've heard that 10% is often the norm - so you're more likely to be travelling more slowly than indicated, obviously so you don't speed. They also over-read more the faster you travel.

And the differences were quite interesting. I set cruise at an indicated 30mph along a long, straight urban road and the RNS-E reported a steady 27mph. A few days later, while thundering down the new bit of A120 past Stansted airport, I set cruise at what the needle said was 90mph, but the RNS-E said was just 83mph. Still around 10% then.

This doesn't mean we should all go out and weld our right foot to the floor, thinking an extra few mph will be safe. But I have always thought the A3 felt slower than the speedo indicated, and this kind of proves it.
 

Vertigo1

Registered User
My last one over-read by 10% but the new one is much better, only 2mph out at 90.
 

coupe-se

Registered User
I've always wondered if this means that the mileometer on the car is over reading by 10% as well? That would also throw out mpg figures!! :scared2:

My A3 speedo is pretty accurate commpared to my Tom Tom and alot more accurate than others cars I've owned. :icon_thumright:
 

rowansbank

Registered User
My A3 speedo is pretty accurate commpared to my Tom Tom and alot more accurate than others cars I've owned.

Same here - on 17" wheels an indicated 72/73 on the speedo shows as 70 on the sat-nav. And at lower speeds the discrepancy is much smaller so that 40 on the speed is not much less. My Navara OTOH shows 75 at a sat-nav 70 which is much more the usual way (C class MB, XTrail same error).
 
My RNS-E admittedly was a retrofit and hasn't been coded yet with VAGCOM. I wonder if this has anything to do with the 10% difference I'm seeing? Can't see how - the RNS-E functions perfectly and I guess the speedo is still a mechanical device. Will do some more testing.
 

Dandle

Registered User
benw123 said:
My RNS-E admittedly was a retrofit and hasn't been coded yet with VAGCOM. I wonder if this has anything to do with the 10% difference I'm seeing? Can't see how - the RNS-E functions perfectly and I guess the speedo is still a mechanical device. Will do some more testing.

I believe after a retrofit that you do need to set the wheels and tyres sizes you are using into the rns-e. This could be where you difference is. Have a read up on it as im not totally sure though.
 

cjp2k

Registered User
dandle said:
I believe after a retrofit that you do need to set the wheels and tyres sizes you are using into the rns-e. This could be where you difference is. Have a read up on it as im not totally sure though.

This is correct, Craig coded my wheel size into the navi with his vagcom
 

rowansbank

Registered User
Tried this on mine - RNS seems to be about 1 mph less than speedo...

The satnav speed reading can be misleading in this regard because it is actually measuring the vehicle's speed between geographical locations on a virtual map without considering their respective altitudes of those locations whereas because the speedo is measuring (in crude terms) the number of turns made by the road wheels on the road surface it is actually taking into account the gradient. That's why the police sometimes say you should not rely on the satnav readout to prevent you from speeding but I suspect the gradient has to be fairly steep to make a significant difference.
 

Dandle

Registered User
The satnav has a speed input into it from the speedo/ecu I would think its working out speed from that input not from the gps.
 

h5djr

Registered User
VCDS Map User
Gold Supporter
With my previous A3 the speedo was about 9% out when compare to the independent GPS in my RoadPilot MicroGo. My current A3 is almost spot-on using the same GPS device.
 

rowansbank

Registered User
The satnav has a speed input into it from the speedo/ecu I would think its working out speed from that input not from the gps.

Sorry I should have made it clear that I was referring to an aftermarket satnav eg TomTom which relies on point-to-point speed calculation.
 

rowansbank

Registered User
The satnav has a speed input into it from the speedo/ecu I would think its working out speed from that input not from the gps.

It's just struck me that, if that is the case, why would the RNS-E readout differ from that of the speedo?:search:
 
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