MPH on Satnav

[Deleted User]

Hello Guys, I'm somewhat new to using a satnav, but I'm curious about something I noticed. Whilst driving, the speed shown on my car speedometer seems to be about 2 - 5 mph higher (depending on speed) than the travelling speed shown on the satnav. I would have thought that the satnav would be a more accurate representation of actual speed; would this be correct? Many thanks for any comments.

Gloomendoom

A speedo is allowed to over read by 10% but not under read.

The GPS is probably more accurate but it will depend on the sampling frequency. With some older units, this was as low as once per second. I believe the newer units sample at around five times a second.

scooby75

Your satnav probably is more realistic of your speed. Speedometers as I understand cannot read less than the speed actually achieved, so in-car speedometers are manufactured to read a couple of mph excess.

But as you point out, your satnav varies in the speed so is probably less realiable. At the end of the day, I would trust my speedometer than may satnav. That's its designed purpose and if anything went wrong (e.g, caught speeding when I wasn't as a not necessarily realistic example), the speedometer in the car would be the reference used in my defence.

[Deleted User]

scooby75 wrote: »

Your satnav probably is more realistic of your speed.
But as you point out, your satnav varies in the speed so is probably less realiable.

Thank you for your reply. I'm trying to determine which is the more reliable instrument, but it seems you are contradicting yourself. Like you, in your first statement, I had thought that the satnav is more accurate, but now I'm not so sure.

Gloomendoom

The speedo works in real time but the satnav will tell you the speed that you were doing last time it did the calculation, which could have been nearly a second ago. If your speed has changed in that time, it will not be a true reading.

I suspect that your speedo is over reading. Compare the two readings at different speeds.

[Deleted User]

Gloomendoom wrote: »

The speedo works in real time but the satnav will tell you the speed that you were doing last time it did the calculation, which could have been nearly a second ago. If your speed has changed in that time, it will not be a true reading.
I suspect that your speedo is over reading. Compare the two readings at different speeds.

Thanks for your reply. I think I'd better rely on the car speedo. I don't want to get caught speeding!!!

londonTiger

my speedo reads higher than it actually is. i think it may be because tyres are thinner (but just about legal). maybe newer tyres willhave more circumference and read speed better

Hintza

Your SatNav will be more accurate generally (unless accelerating/braking hard) for roadworks and average cameras I use the SatNav.

photome

Hintza wrote: »

Your SatNav will be more accurate generally (unless accelerating/braking hard) for roadworks and average cameras I use the SatNav.

So do I, with cruise control on

Mrs_Imp

SatNav won't be so accurate on hills, as they generally see the world as a flat plan.

forgotmyname

Sat Navs are not 100% accurate. Experiment and see how closely it can plot your position.

Sometimes its several feet out, How can something thats only accurate to several meters be 100% reliable for your speed?

Stick to what your speedo says.