Speedo Accuracy

I have just bought a Navman GPS sleeve for my IPAQ (clever toy I might add!). The Navman "reads" your position from satellite fixes every second and is sufficiently accurate (I think) to track speed changes on its display as you walk along - walk faster and it instantly shows a change of speed, and vice versa.

What I found interesting on the weekend was comparing the speed shown on my S60 and that shown on the Navman. The results were constant suggesting that both speed measuring devices were consistant in their reporting.

With a speedo reading of 30 mph the Navman was reporting 27 mph, at (speedo) 34, (Navman) 30, then 70 v 65, 76 v 70, 80 v 73 and 85 v 77.

Are the speedos deliberately set to give low readings? Are they accurate and the Navman not so? If the speedos were calibrated the other way (ie, reading too low), would you have a case against Volvo for speeding tickets?

Speedos are useually incorrect, so they are set low so that you're not doing 75 when you think you are doing 70 and get flashed. I'd think the GPS was the more acurate of the two...

Stu

I thought speedos could have a 10% margin of error. If that's still the case it looks as though yours may be a bit higher.

I imagine reading low is probably better than high though.

Might be worth a letter to Volvo to stir it up and ask them to explain their speedo innacuracies !!!


Mike

[rstrang]

>I thought speedos could have a 10% margin of error. If that's
>still the case it looks as though yours may be a bit higher.
>
>I imagine reading low is probably better than high though.
>
>Might be worth a letter to Volvo to stir it up and ask them
>to explain their speedo innacuracies !!!
>
>
>Mike

I think your speedo is allowed by law to read +10% -0% ie. worst case car is doing 70mph speedo indicates 77mph.

As manufacturers need manufacturing tolerances of + & - they will design their speedo to read +5% this means all speedos will probably read wrong....

This of cause gives you no reason to be caught speeding :-)

Richard

[Peter Milnes]

Do not forget that different tyre sizes have different rolling radii which will affect the speedo reading by a surprising amount. My speedo is more accurate now as I have fitted 195/65/15 size tyres instead of the previous owner's tyres which were 185/60/15 size. I have a small article from one of my IPD catalogues which I think will be worth repeating and putting up on our FAQ section. If this would be of help, Bob, please let me know and I will send you the article.

All the best, Peter

[markas]

Go to an offical rolling road centre and have it calibrated like the police have done.

mark

[Peter Milnes]

The police usually fit their own calibrated speedo unit which is a large round unit with cable operation. A cable take off unit is fitted to the gearboxes that they use.

All the best, Peter

[cootuk]

I thought speedos were best reading slightly high...that way an indicated 70mph would be a bit lower....and the usual motorway indicated 80mph would maybe be only 75mph....

[JackT5]

My T5 is ex-police and speedo wears a sticker "calibrated by irq nottingham", as for accuracy....no idea.

JackT5