speedometer inaccuracy

Noticed today when using the sat nav for 1st time that the speed on sat nav didn't match up with speedo. Now I assume that the sat nav is more likely to be the one that is spot on. At a steady 60 on speedo the sat nav was reading 57 .... when up to 90 (oops) the nav was reading 85.
Is this inaccuracy normal. Also wondered if wheel size affects the speedo ... ie would 16'' rims help bring the speedo back ... or make it worse.
Im not at all concerned .... just interested is all.

The speedo will allways read faster than what your really doing. Not sure why, all though i have been told in the past its so you can't sue the car manufacture if you get a speeding ticket.

Yep, as Simon says...

Also fitting 16" wheels makes the speedo over reading worse.
The 16" wheel with a 215/40 16 tyre fitted is actually slightly smaller than a 15" wheel with a 205/50/15 tyre fitted. :yesnod:

Ah, I see.
As for speeding tickets ... never had any points on my licence to date. I suspect that now I've got Mr Tom to warn me of speed cameras, sods law says Ill get one now. :oops:

rog1963 wrote: Ah, I see.
As for speeding tickets ... never had any points on my licence to date. I suspect that now I've got Mr Tom to warn me of speed cameras, sods law says Ill get one now. :oops:

Has it got speed camera positions on it?

not yet... but when I plugged it into the computer it looked like they were available for download @£14.95. As Im heading up to Notts in the week I think I might invest.

Speedos do over read as standard, but satnavs aren't as accurate as people think either.

so sat navs and speedos over read i wonder if law enforcement cameras do the same :ohmy:

rayb wrote: so sat navs and speedos over read i wonder if law enforcement cameras do the same :ohmy:

They have to be recalibrated regularly to keep them right. :yesnod: :yesnod: .

Which is how you get away with speeding tickets, You ask for the last calibration certificate and 9 out 10 times its out of date.

I think it also depends on satellite lock. If you're locked onto three sats, then the GPS will not be as accurate as its a triangulation equation. Last week I was using a phone app tomeasure speed etc, and at one point was locked onto 11 satellites!! I didint even knwo there were that many GPS sats :lol:

So with 11 points to measure from, the accuracy increases greatly

Just about every geostationary satalite (tv and phone, weather, mapping) that goes up has a GPS module on it. They use it keep the satalite in place, becase they can drift about a fair bit, normally up to a kilometer off target, the gps is locked on to a ground target and uses thrusters to get back into postion, after about 10 years all the fuel is used up and the satalite is allowed to burn up in our atmosphere, there's hundrends of them all together, its getting pretty busy up there, the astra and hotbird satallites (you get sky and freesat from these) have to use thrusters to avoid each other.

GPS is accurate at the magic number of 7 locks at sea level traveling dead flat, soon as you travel up hill or down hill it can't cope. first generation couldn't cope with different altitudes either but current generation are lot better on level travel at different altitudes.

As broon says the more locks you have the greater the accuracy you have.
We are talking a variance of half a foot and 0.5 mph here, its good enough for me.