what gives more accurate speed

[daniel-1986]

i was on the motorway last week using my satnav and wen i looked at the speedo it says 70 but i look at my satnav which tells me how fast im goin aswell and it was sayin between about 65. i know my 15" wheels won't make any difference as the rolling radius was about the same as original my 13" wheels

[cняis]

speedo's over-read by about 10%, in other words your satnav is much more accurate.

[Conker]

Your sat nav will start to become prone to error if you go above 943mph though, so watch out for that.

[takethefifth]

Your sat nav will start to become prone to error if you go above 943mph though, so watch out for that.

  Cheers dude! I'll keep that in mind next time i'm ragging it home! 

[H981CVV]

its nout to worry about, i was driving a 56 plate T5 yesterday back from bicester and the speedo was 3-5 m.p.h above my sat nav so even new cars with standard wheels arnt accurate, i would imagine thats why police give that 10% tolerance with minor speeding offences to allow for this.

[Conker]

Your sat nav will start to become prone to error if you go above 943mph though, so watch out for that.

  Cheers dude! I'll keep that in mind next time i'm ragging it home! 

 

[Dolly]

i had use of a BMW 118d rep mobile, my sat nav was reading around 67 when the speedo was bang on 70.. the margin was pretty much the same at all speeds.. i guess at least theres less chance of getting done for speeding.. say you go through a camera at 35 on the speedo, you may actually only being 30-32

[PDWC]

Doesn't this depend on whether you are going up or down hill? Isn't the Sat Nav reading your progress over the ground from space?

My speedo was way too optomistic (before it broke). Here is a website that can give you speed at various rpm using wheel/tyre size, final drive etc:

www.scirocco.org/grears/

[leavingeasy]

If you ever build a car, the SVA regs state the speedo should overead by c.10% so your results are totally normal.

GPS is very accurate, so learn the error on your speedo and live with it.

Interestingly, assuming you had a 100,000 mile car with a 10% error, your car has actually been serviced one time more often that it needed!!!

[takethefifth]

Doesn't this depend on whether you are going up or down hill? Isn't the Sat Nav reading your progress over the ground from space?

If your Sat Nav is recieving signals from four or more GPS satellites it can factor in altitude as well