Speedometer And Satnav Speed

[Swindon Andy]

I've been suspicious for some time that my speedo is reading faster than real. We cross-checked it on cruise with my partner's portable satnav. With the cruise set to 77, the satnav was reading 71. I make that about 8% optimistic at motorway speeds.

My questions are these, hope this isn't repeating previous thread:

1. Will the satnav reading be pretty accurate?
   
2. Is 8% optimistic fairly normal for these cars and other current makes? My previous car was pretty much was dead on.
   
3. Is the GS3 speedo able to be recalibrated?
   

Hope you guys can advise on this.

[LQA]

Speedometers will err on the high side. I believe they are not permitted to read lower than actual speed in the interests of safety.

SatNavs will be more accurate but there are caveats in that ultimately it is defined by the accuracy of the satnavs software amongst other things.

In order to get the most accurate reading you must drive on a level road in straight line for as long as possible at a constant speed.

GPS satellite transmit a signal every 30 seconds which is why sometimes satnavs show you going 'offroad' when you make a sudden detour. There's more to it than that but you get the gist.

[Swindon Andy]

Thanks, yes got pretty consistent results over long distances.

Can anything be done to recalibrate the speedo other than larger dia tyres?

I don't want to have to do maths every time.

Would the odometer also be optimistic, I wonder? If so, would be 8% shorter service intervals and guarantee periods - all to the dealer's advantage.

[tHomBleached]

Thanks, yes got pretty consistent results over long distances.

Can anything be done to recalibrate the speedo other than larger dia tyres?

I don't want to have to do maths every time.

Would the odometer also be optimistic, I wonder? If so, would be 8% shorter service intervals and guarantee periods - all to the dealer's advantage.

In all the cars I've owned since satnavs have existed, the discrepency you've detailed is pretty much what I've experienced, it's nothing to worry about, as said above, it's better to er on the side of caution.

I would be very suprised if anyone would toy with the calibration of the cluster unit.

I can't see why the odometer would be reading a wrong value though. A distance unit is much easier to measure and therefore more definitive than one involving time (speed)

[Swindon Andy]

I assumed the odo and speedo are both derived from the same input, that's why I wonder about it.

[knitware]

Don't worry about it, really. Enjoy.

[stevie_]

what they said, it's not a conspiracy. every car i've had under reads as well.

[Swindon Andy]

I usually cruise at the max speed limits, where conditions permit. So now I will cruise at 83, which is the 'allowed' 77 real.

I've been told by a cop if you get done for anything under 85 on a clear motoway they are having a bad day, so this should leave a fair margin.