Speed measurements

mikey72

forgotmyname wrote: »

Most satnavs i have used cannot place your position exactly so they are a few feet or even metres out.

How can you say thats more accurate than a speedo?

Yes Yes... Magazine testers use GPS systems to test speed, But they also use road speed measuring equipment to
back that up to make it super accurate. And they dont use £50-£200 satnavs.

£££££.

This comes up every time as well.
Satnavs don't use position to measure speed, they use doppler shift.

Ebe_Scrooge

In a similar vein - I've recently bought a new ( well, second hand but new-ish ) car, and it's got cruise control - woo-hoo ! Anyhow, I tend to stick the CC on at 66mph coming home from work and have a leisurely plod up the motorway. The digital display says CC is set at 66, but the ordinary speedo shows about 69. Not that it matters, but any idea which would be more accurate - surely they're both getting their reading from the same source ?

( I've never had cruise-control before, but I'm getting to like it - I merrily carry on past the many speed-camera-vans they seem to like so much round here, without a care in the world, whilst people all around me are frantically braking as soon as they catch sight ! )

marlot

forgotmyname wrote: »

Most satnavs i have used cannot place your position exactly so they are a few feet or even metres out.

How can you say thats more accurate than a speedo?

In my case, because I set the car on cruise control, and over a longish distance, I compared the satnav speed with the speedo, on a straight, level road.

The satnav is only accurate to a few feet, but over a long distance (even just a few hundred metres) its sufficiently accurate for this purpose.

In day to day driving the speedo reacts faster so is the better inidicator. But having mentally calibrated it, I know approximately how accurate my speedo is at a range of speeds.

marlot

Ebe_Scrooge wrote: »

In a similar vein - I've recently bought a new ( well, second hand but new-ish ) car, and it's got cruise control - woo-hoo ! Anyhow, I tend to stick the CC on at 66mph coming home from work and have a leisurely plod up the motorway. The digital display says CC is set at 66, but the ordinary speedo shows about 69. Not that it matters, but any idea which would be more accurate - surely they're both getting their reading from the same source ?

By law, a speedo is not allowed to under-read your speed. It is allowed to be up to 10% over. So many manufacturers set them to read about 5% faster than the actual speed of the vehicle.

If you use a satnav to check your speed over a straight level road you'll probably find it around 66.

gilbert_and_sullivan

Having run satnav in trucks with calibrated speedo's (which vary up to about 2mph due to tyre wear themselves), without a doubt on the open flat road a good satnav is very accurate.

There will always be the odd fluctuation when they'll throw a wobbly so don't assume a snapshot reading is correct, for instance, my satnav continues to plot my course through a tunnel at the speed it lost the signal then recalculates on emerging, but over a period of use you'll know exactly how far out your car speedo is.

Best way to use the speed function of the satnav is to learn over time what adjustment to make mentally to your car speedometer.